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IC 


8955 



Bureau of Mines Information Circular/1984 




Underground Mine Communications, 
Control and Monitoring 

By Staff, Pittsburgh Research Center 




UNITED STATES DEPARTMENT OF THE INTERIOR 



fflb^^ ^^^^^fe^^-M/t^^ cr^'HA:^^ 



Information Circular]8955 

4/ 



Underground Mine Communications, 
Control and Monitoring 

By Staff, Pittsburgh Research Center 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 




\>H 



(\i" 



1°^ 







Library of Congress Cataloging in Publication Data: 



Underground 


mine communications, control 


and monitoring. 




(Information circular / United States D 
reau of Mines ; 8955) 


epartment of the 


Interior, Bu- 


Includes 


bibliographical references. 






Supt. of Docs, no.: I 28.27:8955. 






1. Mine 
II. Series: 
8955. 


communication systems. I. 
Information circular (United 


Pittsburgh Research Center. 
States. Bureau of Mines) ; 


TN295.U4 


[TN344] 622s [622'. 2] 


83-600288 





<v^ PREFACE 

OO 

"^^ Since 1969, the Bureau of Mines, U.S. Department of Interior, has 
^ sponsored numerous programs aimed at improving methods of underground 
communication. As a result of these research and development programs , 
a wealth of information has been made available to the mining industry. 
Unfortunately, some of this material is highly analytical, and most is 
written in terms best understood by communication specialists. Because 
of the volume of data (over 100 studies have been performed) and its 
highly technical nature, most of the information is not readily avail- 
able for practical application by mine operators. This manual brings 
together relevant data from all previous reports, studies, and other 
sources, and presents these data in such a way that they may be applied 
by the mining industry to improve communications in underground mines. 

This report is intended as a guideline and not as a comprehensive 
documentary of mine equipment. Installation of equipment in a mine 
should be done only by people thoroughly qualified to do such work. 
Installations should follow procedures recommended by the equipment man- 
ufacturer and should comply with good safety practices. All installa- 
tions should also comply with applicable codes and regulations. 



The views and conclusions contained in this document are those of the contractor, 
Collins Avionic Division, and should not be interpreted as necessarily representing 
the official policies or recommendations of the Interior Department's Bureau of Mines 
or of the U.S. Government. 






cD 



iii 

CONTENTS 

Page 

Preface 1 

Chapter 1. — Introduction 1 

1.1 History of Underground Comnunl cat ions 1 

1.2 Productivity and Safety 2 

1 . 3 Constraints on Equipment 4 

1.4 Communication Requirements 5 

1.4.1 The Working Section Crew 5 

1.4.2 The Maintenance Crew 6 

1.4.3 Motormen 6 

1.4.4 Inspectors and Management Personnel 6 

1.4.5 The Dispatcher 6 

1.4.6 Hoist Communications 7 

1.5 Present Communication Systems 7 

1.6 Summary 8 

Bibliography 9 

Chapter 2. — Communication Systems 10 

2. 1 Introduction 10 

2.2 Wired Phone Systems 10 

2.2.1 General Telephone Theory 11 

a. Magneto-Type Telephones 11 

b. Sound-Powered Telephones 11 

c. Paging Telephones 11 

d. Dlal-and-Page Telephones 13 

2.2.2 Distribution Systems 13 

a. Single Pair 14 

b. Figure-8 Multipair Cable 14 

c. Coaxial Cable 15 

d. Multiplex Systems..... 15 

I. Frequency Division Multiplexing 15 

11. Time Division Multiplexing 16 

2.2.3 Telephone Exchanges 17 

a. Manual Switchboard 17 

b. Private Automatic Branch Exchange 17 

c. Computer-Controlled Switches 18 

2.3 Radio Systems 18 

2.3.1 General Radio System Theory 18 

a. Amplitude Modulation 18 

b. Frequency Modulation 18 

2.3.2 Distribution Systems 19 

a. Antenna Theory 20 

1. Half-Wave Dlpole Antenna 20 

II. Quarter-Wave Antenna 21 

ill. Long-Wire Antenna 21 

Iv. Loop Antenna 21 

b. Leaky Feeder Systems 22 

c. Waveguide Propagation 22 

d. Repeaters 23 

1. Fl-Fl Repeater 23 

11. F1-F2 Repeater 24 

2.3.3 Through-the-Earth Radio 24 

2.3.4 Radio Pagers 25 



CONTENTS — Continued 

Page 

2.4 Carrier Current Systems 25 

2.4.1 Trolley Carrier Phone 26 

2.4.2 Hoist Rope Radio 26 

2.4.3 Medium-Frequency Radio , 27 

2.5 Hybrid Systems ,. 31 

2.5.1 Improvements in System Versatility 31 

2.5.2 Dial Phone-Pager Phone Systems 32 

2.6 Other Systems , 33 

2.6.1 Seismic Systems 33 

2.6.2 Stench System , 33 

2.6.3 Hoist Bell Signaling... 33 

2.6.4 Visual Pagers , 34 

2.7 Summary , 34 

Bibliography 35 

Chapter 3. — Solutions to the Communication Requirements 37 

3. 1 Introduction. 37 

3.2 The Mine Entrance 38 

3.2.1 Bell Signaling Systems 39 

3.2.2 Trailing Cable Systems 40 

3.2.3 Radio Systems 40 

3.2.4 Hoist Rope Carrier Current System ., 41 

3.2.5 Hoist Signaling Summary 41 

3.3 Permanent and Semipermanent In-Mine Locations 42 

3.3.1 Single-Pair Pager Phones 42 

3.3.2 Multipair Systems 43 

3.3.3 Multiplexed Systems 44 

3.4 Mining Area ■, 45 

3.4.1 Radio Systems .,,. 45 

3.4.2 Longwall Mining 49 

3 . 5 Haulageways 52 

3.5.1 Trolley Haulage 52 

a. Dedicated Wire 54 

b . Summary 56 

3.5.2 Nontrolley Haulage 56 

a. Leaky-Coax Systems 56 

b. UHF Reflective Techniques in Underground Mines 59 

c. Dedicated-Wire Radio Systems 60 

d. Wireless Radio System 61 

i . Interference 61 

ii. Signal Attenuation in the Haulageway 63 

iii. Signal Attenuation Around Corners 63 

3.5.3 Belt Haulage 64 

3.6 Special Requirements 64 

3.6.1 The Roving or Isolated Miner 64 

a. One-Way-Voice (Pocket) Pagers 65 

3.6.2 Mo to rman- to- Snapper 67 

a. Telephone and Trolley-Carrier Phone System 67 

b. Walkie-Talkie System 68 

3 . 7 Emergency Communications 69 

3.7.1 Detecting and Locating the Trapped Miner 69 



CONTENTS—Continued 

Page 

3.7.2 Refuge Shelter 72 

3.7.3 Rescue Team Comminicatlons 73 

3.7.4 Medium-Frequency Rescue Systems 73 

a. Specific Application of MF Communications to Rescue Teams... 74 

b. System Concepts 75 

c. Location and Communications Systems for the Rescue of 

Trapped Miners 76 

d . Performance Data 79 

3.7.5 Emergency Warning Systems 79 

Bibliography. 82 

Chapter 4. — Computerized Mine Monitoring 85 

4. 1 Introduction. 85 

4.2 Uses of a Mine Monitoring System 85 

4.3 Petitions for Modification 87 

4.4 Technical Factors 88 

4.4.1 Sensors 88 

4.4.2 Telemetry 89 

4.4.3 Reliability 89 

4.5 Commercially Available Mine Monitoring Equipment 90 

4.5.1 Introduction 90 

4.5.2 Telemetry-Analysis and Display Systems 91 

a. Systems Suppliers 93 

b . Summary 98 

4.5.3 Sensors , 99 

a. Air Velocity Sensors 100 

b. Methane Sensors 101 

c. Carbon Monoxide Sensors 103 

d. Dust sensors 104 

4.6 Existing Mine Monitoring Systems 104 

4.6.1 U.S. Underground Coal Mines 105 

4.6.2 U.S. Underground Metal-Nonmetal Mines 115 

4.6.3 Foreign Mines 115 

References. 119 

Chapter 5. — Communication System Design and Improvement 121 

5.1 Introduction 121 

5.2 New Phone System Design 121 

5.2.1 Wired Phone Systems 122 

a. Single-Pair Systems 122 

b. Multlpair Systems 124 

c. Multiplexed Phone Systems 126 

5.2.2 Cable Selection 127 

5.2.3 Summary 130 

5.3 Improving Existing (In-Place) Phone Systems 131 

5.3.1 Trolley Carrier Phone Systems 131 

a. Isolating Loads at the Carrier Frequency 132 

i. Rectifiers 133 

ii. Heaters 136 

iii. Vehicle Lights 137 

iv. Other Loads 137 

b. Using a Dedicated Wire 138 



vi 



CONTENTS—Cont inued 

Page 

c. Using a Remote Transceiver 139 

d. Summary 140 

5.3.2 Improving Telephone Systems 140 

a. Loopback Methods 141 

b. Sectionalizing the Underground Network 142 

5.3.3 Summary 142 

Bibliography 143 

Chapter 6. — Installation Techniques 144 

6.1 The Basic Philosophy 144 

6.2 Pager Phone Installation 144 

6.2.1 Mounting 144 

6.2.2 Connections 145 

6.2.3 Batteries 145 

6.2.4 Fuses 146 

6.2.5 Amplifier Loudness 146 

6.3 Phone Lines and Transmission Cables 146 

6.3.1 Phone Lines 146 

6.3.2 Leaky Feeder Cable 147 

6.4 Carrier Phone Installation 147 

6.4.1 The Dispatcher Location 147 

6.4.2 Vehicle Installations 150 

6.5 Carrier Current Hoist Phone 154 

6.5.1 Cage 154 

6.5.2 Holstroom and Headframe 155 

6.6 Summary 155 

Bibliography 158 

Chapter 7. — ^Maintenance. 159 

7 . 1 General 1 59 

7.2 Preventive Maintenance and Inspections 159 

7.2.1 Cables 159 

7.2.2 Pager Phones 159 

a. Listen Circuit 159 

b. Page Circuit and Talk Circuit 159 

c. General Comments 160 

d. Battery Condition 160 

e. Battery Testing 160 

7.2.3 Carrier Phones 161 

a. Microphone 161 

b. Batteries 162 

c. Wet Cell Maintenance 163 

d. Gelled Electrolyte Battery 163 

e. Troubleshooting on the Vehicle 163 

f. Mapping Signal Levels 166 

7 . 3 Summary 168 

Bibliography 167 

Appendix A. — Communication System Examples 170 

Appendix B. — Federal Regulations 197 

Appendix C. — Equipment Suppliers 202 

Appendix D. — Glossary of Terms 208 



CHAPTER 1.— INTRODUCTION 



1.1 History of Underground 
Communications 

Although the technology involved in 
removing material from below the earth's 
surface has a long history, communication 
systems in underground workings are rela- 
tively new to the industry. Communica- 
tion equipment did not begin appearing in 
underground mines until the early 1900' s. 
Figure 1-1 shows a miner using a Western 
Electric standard telephone set for 
underground mines in 1913. These early 
phones were essentially the same as those 
used aboveground, except that they were 
enclosed in cast-iron boxes as protection 
against moisture, acid fumes, and gases. 

In the 1950' s, the Chesapeake and 
Potomac Telephone Co. of West Virginia 
introduced a telephone set for use in 
explosive atmospheres that was designed 
around the philosophy of explosion 
containment. To contain an explosion 
within the telephone set required a 
39-pound casing, which greatly limited 
portability. 




FIGURE !•!. - Coal miner talking on an under- 
ground telephone in 1913. 



Both of these telephone sets 
required "pipe" or conduit, not a very 
practical item for a 100-square-mile coal 
mine. A conmion thread prevalent in the 
first 40 years of design and development 
is that the primary effort was placed on 
the telephone set. 

In the early 1970' s, as work began 

on the design of modern conmiunication 

systems for use in underground mine 

environments, the following requirements 
were established: 

1. Must meet intrinsic safety 
standards. — This applies not only to 
coal mines, but to other explosive 
environments. 

2. Must be compatible with the 
environment. — The system must with- 
stand dust, moisture, and corrosive 
conditions. 

3. Must be rugged in structure. — 
The system must withstand maintenance by 
10-inch pliers and 4-pound hammers, as 
well as impact from a falling piece of 
roof. 

4. Must be size-flexible. — Whatever 
is built must be sized for the small 
operator as well as the large companies. 

5. Must be a total system in 
design. — The system must be intrinsically 
safe, including cable, power supply, and 
station set. 

6. Must work with present tele- 
phone system. — The system should be com- 
patible with aboveground communications 
already in place; it should not be the 
single cause of change in aboveground 
communications . 

7. Value-added pricing. — The system 
should be reasonably priced so that sav- 
ings created by its introduction will 
more than offset the installation cost. 



8. Full-service full-support con- 
cept. — The service offering the system 
must provide for maintenance, training 
(initial and continuing), route design, 
and transmission engineering. 

In recent history, the most common 
method of underground mine communications 
consisted of louds peaking- or paging- 
type telephones, or alternatively, mag- 
neto ringing telephones. In most cases 
these phones were connected on a common 
party line with one telephone for each 
working section and additional phones at 
other key locations, such as maintenance 
shops, both underground and on the sur- 
face. As mining operations became more 
mechanized, underground rail haulage sys- 
tems were developed. Eventually, these 
were driven by electrically powered loco- 
motives, and trolley carrier current com- 
munication systems were developed. While 
a few mines have begun to use dial-type 
telephones underground, their use to date 
has been very limited. It is expected, 
however, that more and more mines will 
install dial-and-page telephones and 
radio-type communication systems in the 
future. Today's rail transportation sys- 
tems are typically equipped with carrier 
current phones to provide communications 
between vehicles and between vehicles and 
a central dispatcher. 

Early shaft communications consisted 
of bells or whistle signaling systems. 
Today these systems have been replaced 
with radio and carrier current systems 
that allow two-way voice communication to 
and from personnel in the cage. 

It is recognized tnat presently 
available mine communication systems need 
improvement. Research and development 
programs are continually being conducted 
by the Bureau of Mines , the mining indus- 
try, and equipment manufacturers to 
improve communication technology and 
techniques. The primary objective of all 
of these programs is to increase the pro- 
ductivity of the mining industry and 
increase the safety of miners. 



1.2 Productivity and Safety 

Underground mining operations, like 
other industrial enterprises, are tied 
together by communications. Adequate 
communication within a mine and between 
the surface and the underground work sta- 
tions is a vital part of the proper 
operation of any underground facility. 
This communication capability is not only 
an important factor in the concept of 
safety, but also is an aid to the day- 
to-day operations and the task of 
extracting and moving the product to the 
surface. If a rapid, accurate flow of 
data is automatically and continuously 
presented to management, then decisions 
can be made sooner and more accurately. 
Some people consider the operation of a 
mine to be a relatively static operation 
planned many months, even years, in 
advance. However, the productive time 
per shift and productivity trends indi- 
cate that a large amount of waste time 
accumulates due to unpredictable daily 
events. If management were aware of 
breakdowns within minutes, attention 
could be more quickly focused on solving 
the problems, thereby increasing long- 
term production. 

Safety can certainly be enhanced by 
accurate fast communication channels. 
Quicker medical assistance, faster evalu- 
ation of the situation underground, and 
accurate location of problems will be 
direct benefits. In addition to these 
obvious advantages, remote monitoring and 
control of equipment and conditions 
underground will allow management per- 
sonnel to prevent accidents and other 
causes of production losses. 

A study of 9,300 injury-causing 
accidents that occurred in underground 
bituminous coal mines during 1974 found 
that the financial cost totaled almost 
$57 million, or an average of $6,100 per 
accident. This figure did not include 
the intangible cost of reduced efficiency 
in fellow workers resulting from the 
accident. The study detected a marked 



tendency of mine crews in or near the 
area where the accident occurred to slow 
down their pace of operations for a 
period of time, especially after a seri- 
ous accident; consequently, production 
would drop. Such a drop was in addition 
to the accident itself, and to the time 
required to clean up after the accident. 
About 41%, or an estimated $23.6 million, 
of the total (current and future) cost of 
underground coal mine accidents in 1974 
was borne by mining companies in the form 
of compensation payments to accident vic- 
tims and survivors, lost coal production, 
and the expense of investigating the 
accidents. Any devices or techniques, 
including good communication systems, 
that will reduce accidents will quickly 
pay for themselves. 

The evolution of mining technology, 
including underground communications, is 
inevitable as coal assumes a more sig- 
nificant role in the national energy 
plan. Another factor that will aid this 
development is the application of modern 
remote control and monitoring methods to 
increase production. Advances in remote 
monitoring and control of equipment and 
conditions underground will involve 
increased use of computers. Mine moni- 
toring systems using a coii5)uter have 
already been installed in some mines. 
Basically, these systems consist of 
sensors placed at strategic locations, 
data relay stations, and a digital com- 
puter in the mine office. The computer 
is programed to process the data, deter- 
mine when and where abnormal conditions 
exist, and alert mine personnel. 

Six types of sensors in common use 
detect methane, carbon monoxide, tempera- 
ture, relative humidity, airflow, or dif- 
ferential air pressure. Systems can 
include an electronic display of the mine 
layout, electric typewriters, and video 
display screens connected to the com- 
puter. The computer receives data from 
the remote stations in the form of elec- 
trical signals, which are translated into 
numerical measurements and checked 
against a set of standards to see if all 
factors are within normal limits. If any 
factors at a remote station exceed normal 



limits, an alarm may be initiated. At 
the same time, the electric typewriter 
connected to the computer types out 
either a warning or a danger message to 
specify what is wrong and at which sensor 
the trouble is being detected. 

In the United Kingdom, a commercial 
mine monitoring and control system, MINOS 
(Mine Operating System), is being devel- 
oped. The heart of the system is a digi- 
tal computer (fig. 1-2) located within 
the system control center. This computer 
is programed to obtain various data from 
remote sensors and monitor the satis- 
factory operation of the mine, giving 
audible and visual alarms as well as 
initiating automatic shutdown of equip- 
ment when necessary. Control of the con- 
veyor system including startup, shutdown, 
and feed rate is also possible. Print- 
outs and daily summaries of specified 
information are outputted automatically 
or are available on demand by key- 
board commands entered at the system 
center. Remote control and monitoring 
of surface facilities and preparation 
plants as well as underground equipment 
such as pumps and fans can also be 
acconqjlished with the computer-controlled 
systems. 

Any remote monitor and control sys- 
tem is relatively sophisticated, and an 



VISUAL DISPLAY SCREENS 



INDICATOR LAMPS 




CONTROL PANEL 

FIGURE 1-2. - MINOS system center. 



economic analysis of the tradeoffs 
involved in the mechanization is 
required. However, minewide remote moni- 
toring and control, coupled with an 
effective communication system, is one of 
the tools that can be used to increase 
safety and productivity in underground 



1.3 Constraints on Equipment 

The environmental constraints on 
equipment expected to operate in any 
underground mine are severe. Equipment 
designed for surface operation, even if 
it would work underground, could not be 
expected to last for any length of time 
in the harsh mine environment. 

Equipment must be protected from, or 
imnune to, high-moisture atmospheres (0 
to 100% humidity levels) and remain oper- 
able over wide temperature ranges. Dust 
can be expected to clog airflow passages, 
plug relay contacts, and cause switches 
to stick. Dust accumulation on elec- 
tronic components can also cause heat 
buildup and even "short circuits." Some 
mine atmospheres are highly corrosive, 
and equipment in these mines must be con- 
structed from materials that are resist- 
ant to the corrosion, or else protected 
from it. The very dry atmosphere in some 
mines causes gaskets and seals to quickly 
dry out and start leaking. Static elec- 
tricity in these mines can also pose a 
problem for certain types of solid state 
electrical circuits. 

In addition to the environmental 
constraints, physical considerations must 
be taken into account. Space is often at 
a premium in underground mines, espe- 
cially in low coal seams. Communica- 
tions, control, and monitoring equipment 
must be small in size and should be light 
in weight. Because it must exist in 
close proximity with heavy mining machin- 
ery, communication equipment must also be 
ruggedly designed and shock protected. 
Reliability and ease of maintenance are 
other reasons why most equipment designed 
for surface operation cannot be utilized 
in underground mines. 



Much of the communication equipment 
in coal mines today is located in venti- 
lated areas where there is less likeli- 
hood of an explosion. However, if a ven- 
tilation system is required to control 
the gas or dust hazard and basic commini- 
cations must be maintained in the event 
of a ventilation failure, then this com- 
munication equipment must not be capable 
of generating an ignition spark. Thus, 
much of today's mine communication equip- 
ment carries a "Permissibility" label 
granted by the Mine Safety and Health 
Administration. To achieve the permissi- 
bility rating without explosion-proof 
packaging, all sources of sparks must be 
controlled by limiting voltages, cur- 
rents, and the amounts of stored energy — 
such as in batteries, capacitors, and 
inductors — to safe levels. Such a device 
is defined as "intrinsically safe." 

The amount of spark current in a 
resistive circuit required to ignite 
a methane-air mixture varies with the 
open-circuit voltage. Guidelines to 
safe operating limits, such as shown 
in figure 1-3, indicate a hazard at 




MINIMUM IGNITION CURRENT lAMPERESI 

FIGURE 1-3. - Typical ignition hazard curve. 



2 amperes in a 20-volt resistive circuit. 
However, no greater hazard exists with 
10 amperes in a 12-volt resistive cir- 
cuit. When reactive circuits (circuits 
with inductors or capacitors) or energy 
storage elements (batteries) are con- 
sidered, limits on safe circuit design 
are even tighter. These safety consider- 
ations have directed the design of all 
the "permissible" communication devices 
on the market today. One of the most 
recent pager-type phones does not contain 
any inductive components other than the 
speaker and handset and uses only a 
single 12-volt battery. 

In gassy mines, only permissible or 
intrinsically safe communication equip- 
ment should be acceptable for use in all 
locations under all conditions. 

The permissible rating requirement 
rules out use of most systems designed 
for surface applications. For instance, 
a permissible rating cannot be given to 
a standard telephone connected to a 
telephone switchboard because of high 
voltages (48 volts dc while on hook and 
as much as 120 volts ac while ringing) 
and use of many highly inductive devices 
in the circuit. While placing the tele- 
phone in an explosion-proof housing could 
be considered as a means of making 
it "permissible," the cost would be 
unreasonable. 

Other restrictions on communication 
equipment used underground are specified 
in the U.S. Code of Federal Regulations 
(See appendix B of this manual.) 

1.4 Communication Requirements 

In an ideal communication system an 
individual should be able to initiate and 
receive calls regardless of his or her 
position in the mine. To accomplish 
this, more than one communication system 
may be required. Communication require- 
ments are more readily defined by sep- 
arating the underground personnel in 
present-day mine operations into four 
functional groups: 



Working section crew 

Maintenance crew 

Motormen 

Inspectors and management personnel 

The position of dispatcher is con- 
sidered separately because he or she gen- 
erally coordinates the communications as 
well as the haulage traffic. 

In small mines and belt-haulage-type 
mines the conmiunication center may be the 
responsibility of the hoist engineer, the 
supply man, or the maintenance foreman. 
Because hoist communication requirements 
are unique, they also can be treated 
separately. 

1.4.1 The Working Section Crew 

Under normal operating conditions 
the section forman communicates by fixed 
phone to the shift foreman to request 
supplies and maintenance services and to 
file periodic productivity reports. 
Under emergency conditions he must be 
able to request medical aid for personnel 
and report hazardous conditions in his 
area. 

The high acoustic noise level cre- 
ated by the mining machinery greatly 
reduces the effective communications 
between the foreman and his crew. This 
noise also interferes with the foreman 
receiving calls. Often a motorman must 
deliver a call-in message to the foreman 
when he is transferring hauling cars in 
his section. Some belt haulage mines 
must even resort to turning off the con- 
veyor system, thereby causing all the 
section foremen to call in. Communica- 
tion requirements of the section crew can 
be satisfied by loudspeaking pager phones 
that can easily be moved to keep them 
within hearing range, or by some form of 
radio link (two-way radio, pocket pager, 
or "beeper"). 



1.4.2 The Maintenance Crew 

Unlike a working section crew, the 
maintenance crew is spread throughout the 
mine. The maintenance foreman must be 
able to receive repair requests and dis- 
patch his crews for both emergency and 
scheduled repair work. He should also be 
able to maintain communications with the 
individual crew members while they are in 
transit or after they have arrived at the 
repair site. The dispatcher may provide 
assistance by routing messages for equip- 
ment repair and parts to the foreman from 
the maintenance crew. 

Wireless mobile communication equip- 
ment, linking the maintenance foreman and 
his crew together, would be ideal for the 
above tasks. However, any portable radio 
equipment used must be small and light- 
weight because crew members already have 
much to carry. 



(Newer phones have backup batteries 
installed in each phone so this may not 
be a problem. ) 

In spite of these drawbacks, carrier 
phone systems usually meet the require- 
ments of the motormen, except for an 
emergency that severs the trolley wire or 
otherwise removes power from the wire. 

In mines without tracked-trolley 
haulage systems, a radio link must be 
established to allow motormen to remain 
in contact with one another. 

1.4.4 Inspectors and Management 
Personnel 

These people are underground pri- 
marily to observe mine conditions and 
personnel. They should be able to stay 
in continuous contact with the communica- 
tion center for the following reasons: 



1.4.3 Motormen 

The motormen are responsible for 
coal or ore haulage and the delivery of 
men and supplies to the working sections. 
Rights-of-way and the disposition of 
haulage cars must be known to all motor- 
men to avoid accidents. In mines using 
tracked-trolley haulage, activities can 
be coordinated by a trolley wire (car- 
rier) phone system. These are known as 
"carrier" systems because the communica- 
tion is "carried" on a wire not intended 
for communication, in this case the 
trolley line; the term "carrier," how- 
ever, refers to the technique, not the 
wire itself. This single-channel network 
keeps the dispatcher and all motormen in 
continuous contact with one another. 
This phone system also allows the dis- 
patcher to notify all motormen of any 
mine emergency. The two drawbacks to 
this system are 

1. Dead zones, which are sections 
of track where the phone is inoperative 
owing to excess electrical noise or 
excess attenuation of signal strength. 

2. Trolley wire power failures, 
which cause the phones to go dead. 



To be informed of any emergencies 
that might arise. 

To keep the center informed of their 
location. 

To receive calls from other parts of 
the mine. 

These requirements could be satis- 
fied by an effective, extensive wireless 
mobile communication system. A vehicle- 
mounted system may be sufficient in some 
cases, such as the trolley carrier phones 
in track haulage mines. 

1.4.5 The Dispatcher 

The dispatcher's location in some 
mines has developed into a communication 
center for all underground operations. 
He is in direct contact with all motormen 
via the trolley wire phone system, and 
directs all vehicle traffic in the mine. 
In some mines, he also controls the fixed 
phone circuits via a small switchboard. 
He locates personnel by the paging phones 
or by relaying messages through the 
motormen to the sections. He serves as 
the human coupler between the different 
phone systems, and he is in the best 



position to quickly notify all under- 
ground personnel of any emergency 
condition. 

If this evolution continues, the 
dispatcher's job will expand to the point 
where he may become overworked. For 
safety and productivity reasons the voice 
traffic control and the monitoring func- 
tion of the dispatcher's job cannot 
interfere with his prime responsibility 
of vehicle traffic control. Therefore, 
it may be desirable to transfer these 
responsibilities to other personnel or to 
automatic dialing and alarm equipment. 
For this reason some mines have estab- 
lished a separate communication system 
"operator" position. This person has 
responsibility for voice traffic control, 
and also for monitoring environmental 
conditions in the mine. The operating 
conditions of the haulage and mining 
equipment could also be monitored from 
the communication center. 

1.4.6 Hoist Communications 

A hoist-shaft communication system 
should satisfy the requirements for com- 
munication throughout the full travel 
of the cage, providing two-way voice com- 
munication between the cage and the 
hoistman. The system should also 
allow for shaft-inspection communication 
between the inspector and the "hoistman. " 
For the modern, automated shaft, signals 
are also required for a slack-rope indi- 
cation, to permit selection of level, 
enable interface with interlocks, and jog 
for exact position at any level or shaft 
station. Cage equipment must be small 
and capable of being located so that it 
cannot be damaged by any of the various 
uses of the cage such as transporting 
supplies and machinery into the mine. 

Additional microphone-speaker sta- 
tions should be located on each cage 
level when multilevel "man trip" cages 
are used. Until recently, bell signaling 
was the only form of hoist-shaft communi- 
cation. A disadvantage of the bell sig- 
naling system is that communication 
with the cage when it is between levels 



is impossible. This deficiency is espe- 
cially crucial during shaft inspection or 
repair, where movement of the cage must 
be controlled precisely. Today, however, 
equipment is available that allows two- 
way voice communication between persons 
in the hoist cage and the hoistman or 
other locations at the shaft top or 
bottom. 

Reliable hoist-shaft communication 
should be considered as a vital part of 
the overall communication system, espe- 
cially during or following an accident or 
disaster situation. Experience has 
taught that the hoist often becomes a 
bottleneck during rescue or evacuation 
operations, and good communication to and 
from the cage is essential. 

1.5 Present Communication Systems 

Communication systems currently used 
in many mines generally consist of two 
systems, the trolley wire carrier phones 
and the fixed pager phones. The trolley 
wire channel must be a party line to keep 
all motormen informed of one another's 
location. The pager phone system is 
often divided into multiparty circuits 
which are controlled by the dispatcher. 
For example, one mine studied used eight 
party line phone circuits terminated at a 
simple switchboard in the dispatcher's 
office. A logical partitioning of the 
pager phone network into a multichannel 
private line system would be to give each 
working section a separate line, or at 
the most two sections per line, and have 
one common line for all haulageway 
phones. Individual section phone lines 
would eliminate peak traffic demand dur- 
ing production reporting time and provide 
the section foreman with a private line 
during an emergency situation. Other 
phone lines could be used for monitoring 
the environment and equipment. Based on 
data and survey results to date, it 
appears that about two to eight phone 
lines, depending on activity of the mine, 
would be sufficient to provide efficient 
service for the working sections and also 
provide a common private line for the 
haulageways. 



1.6 Summary 

The need for effective communication 
between locations underground and between 
underground and surface locations has 
been recognized for some time. Unfor- 
tunately, the equipment to totally sat- 
isfy these requirements in underground 
mines has only recently become available. 
Part of the reason for lack of equip- 
ment can be blamed on the uniquely 
harsh environment present in underground 
mines. 

Despite past deficiencies, equipment 
is now available to meet most of the 
requirements for effective underground 
comnunication systems. The operational 
requirement of the ultimate system may be 
simply stated as follows: Each indi- 
vidual should be able to initiate and 
receive communications regardless of his 
location within the mine. In practice, 
this ultimate requirement will normally 
be modified. The size and age of a mine, 
operating conditions, and economic con- 
siderations will affect the degree to 
which a system fully meets this ultimate 
requirement. 

Vehicles operating on rails within a 
mine should have a communication unit 
mounted in every powered vehicle 
(required in some States). Operational 
safety is increased when there is 
an intervehicular communication system. 
Operators can report to each other or to 
a central dispatcher, thereby reducing 
the chances of a collision. Carried to 
the ultimate, the central dispatcher can 
control the movement of all vehicles at 
all times. 

Often there is a need to call a man 
who is not near any vehicle or phone. As 
a minimum, some form of paging capability 
should be included in the overall mine 
phone system that can be used to tell a 
called party to go to the nearest phone 
and return the call. The ability to com- 
municate with men underground, no matter 
what their location, is essential to the 
efficiency of any mine. Safety and pro- 
ductivity are directly related, and both 
depend upon good communications. In 



addition to the obvious advantages of 
reliable and effective communications, 
there are intangible benefits that may 
not be recognized: 

1. The general attitude of the 
underground workforce will be improved If 
they know that they are not "cut off" 
communicationwise from the surface. 

2. Mines with good communication 
systems should be able to more effec- 
tively compete in the labor market. High 
turnover rates, which are costly owing to 
training requirements, will be reduced. 

3. Mines with effective comminica- 
tion systems and good safety records are 
usually subjected to inspections less 
frequently, and since studies have shown 
that production rates decrease when it is 
known that inspection personnel are on 
the property, the effect on production 
should be good. 

Adequate communications within a 
mine and to the surface is a vital part 
of the proper operation of an under- 
ground facility. This communication 
capability is not only an important 
factor in the concept of safety precau- 
tions, but also an aid to the day-to-day 
operations and the task of moving the 
mined product to the surface. The mining 
industry exists to bring the product out 
and to bring it out safely and econom- 
ically. Adequate communication is one of 
the tools available to assist in this 
task. 

In a like manner, a judicious choice 
of remote monitoring and control of 
parameters in the underground environment 
and on selected machinery will yield a 
cost savings in production and augment 
safety. Many man-hours and dollars can 
be saved by knowing conditions before 
they become a problem. Situations that 
could be disastrous can be predicted and 
proper solutions implemented before the 
disaster occurs. Proper environmental 
and machine monitoring is another key 
to safer, more productive underground 
mining. 



BIBLIOGRAPHY 



1. Betsh, K. W. , G. Trace, and P. B. 
Day. A Permissive Dial/Page Telephone 
for Coal Mine Communications. Proc. 2d 
WVU Conf. on Coal Mine Electrotechnology , 
Morgantown, W. Va. , June 12-14, 1974, 
pp. 8-1—8-13. 

2. Collins Radio Co. (Cedar Rapids, 
Iowa). Research and Development Contract 
for Coal Mine Communication System, Vol- 
ume I, Summary and Results of System 
Study. BuMines OFR 69(l)-75, Novem- 
ber 1974, 46 pp.; available from NTIS 
PB 244 169. 

3. Drake, J. L. and J. M. Sticklen. 
An Industrial Communications System, 



Paper in Underground Mine Communications. 
1. Mine Telephone Systems. BuMines 
IC 8742, 1977, pp. 27-41. 

4. Lagace, R. L. , W. G. Bender, J. D. 
Foulkes, and P. F. O'Brien. Techni- 
cal Services for Mine Communications 
Research. Applicability of Available 
Multiplex Carrier Equipment for Mine Tel- 
ephone Systems. BuMines OFR 20(1 )-76, 
July 1975, 95 pp.; NTIS PB 249 829. 

5. Parkinson, H. E. Mine Communica- 
tions. Proc. 2d WVU Conf. on Coal Mine 
Electrotechnology, Morgantown, W. Va. 
June 12-14, 1974, pp. 5-1—5-5. 



10 



CHAPTER 2. — COMMUNICATION SYSTEMS 



2. 1 Introduction 

Any coniniinication system requires at 
least three elements in order to func- 
tion: a transmitting device, a receiving 
device, and a transmission line or propa- 
gation medium. Even the device children 
use, tin cans connected with string, con- 
sists or these three elements. One 
speaks into one can (transmitter), which 
vibrates at the same frequencies as the 
voice. The string (transmission path) 
picks up the vibrations of the can and 
carries them along its entire length. 
The other can (receiver) detects the 
vibration and reproduces the original 
sounds to a lesser extent depending on 
distance, tightness of string, type of 
string, etc. All communication systems 
depend on these three elements: trans- 
mitter, transmission path, and receiver. 

Communication systems can be divided 
into three fundamental categories: wired 
phone systems, radio systems, and carrier 
current systems. Sections 2.2, 2.3, and 
2.4, respectively, describe these systems 
and explain the basic principles of how 
each works. 

Hybrid systems are those systems 
that use various combinations of 
the three basic comminication meth- 
ods. Hybrid systems are described in 
section 2.5. 

There are some other methods of sig- 
naling (stench warning, bell signaling, 
etc. ) that can be used in underground 
mines to transmit or convey information. 
These systems, although they cannot be 
considered true communication systems 
since they do not provide voice or even 
two-way communication, are briefly 
described in section 2.6. 

2. 2 Wired Phone Systems 

Wired phone systems are all those 
that depend on a wire connection between 
phones with the wire carrying the voice 
signals. Figure 2-1 is a diagram of two 
typical wired phone systems. The top 



panel shows a simple single pair party 
line system. In this system each phone 
is connected to a common pair of wires, 
and a person speaking into one phone will 
be heard at all the other phones on the 
line. The bottom panel shows a multipair 
private line phone system. In this type 
of system, each phone is connected by its 
own individual pair of wires to a central 
switch or telephone exchange. To estab- 
lish a call between two phones in this 
system, the lines between the two phones 
must be connected (switched together) 
within the telephone exchange. 

In early exchanges, the connections 
were made manually by an operator. These 
exchanges are called Private Branch 
Exchanges (PBX's). Today, equipment 
within the exchange can automatically 
connect each phone to any other phone in 
the system. These exchanges are called 
Private Automatic Branch Exchanges 
(PABX's). There are many different types 
of automatic exchanges. Some utilize 
switches to physically make each connec- 
tion according to the number dialed. 
Other, more advanced exchanges are com- 
pletely solid state and may even be com- 
puter controlled. 




DISTRIBUTION 
SYSTEM 



al. SINGLE PAIR (PARTY LINE) SYSTEM 




DISTRIBUTION 
SYSTEM 



bl. MULTIPAIR (PRIVATE LINEI SYSTEM 



FIGURE 2-1. - Wired phone systems. 



11 



Telephone exchanges are described in 
section 2.2.3. The various types of 
phones in use today and a description of 
how they operate is given in sec- 
tion 2.2.1. Distribution systems are 
described in section 2.2.2. 

2.2.1 General Telephone Theory 

There are many different types of 
phones in use today, including 

Magneto type 

Sound-powered 

Paging 

Dial 

Dial -and-p age 

These phones and the basic princi- 
ples of how they operate are described 
in the following paragraphs. 

2.2.1a Magneto-Type Telephones 

One of the earliest types of under- 
ground communication instruments was the 
magneto phone, also known as the crank 
ringer phone. These phones consisted of 
a transmitter, receiver, hookswitch, 
ringer, battery, and hand generator (mag- 
neto). As the spindle handle was turned, 
80 to 100 volts at 15 to 18 cycles per 
second was produced by the magneto. This 
current caused the other phones to ring. 
Once the called phone was answered, 
talking power was supplied by battery 
voltage. 

Magneto phones were connected in 
party line fashion with a code of short 
and long rings to identify the called 
station. Some mines still use this type 
of system. However, as mines expanded in 
size, the system proved to lack adequate 
signal strength to power a large number 
of phones. 

2.2.1b Sound-Powered Telephones 

A sound-powered set is one that pro- 
vides a means of voice communications 
with the use of no energy except that 



furnished by the speaker's voice. These 
phones have highly efficient transmitters 
and receivers for converting voice into 
electrical signals. 

The sound-powered handset is compar- 
able in size and appearance to the famil- 
iar battery-powered handset. Two of 
these phones were usually connected 
by a single line to constitute an 
intercom circuit. Such a circuit will 
reproduce speech with reasonable good 
quality for short distances in quiet 
surroundings. 

For special communication applica- 
tions requiring exceptionally rugged and 
durable sets for private comminication 
purposes, and particularly when the line 
loops are short, sound-powered sets are 
well adapted. 



their prin- 
as indepen- 
simplicity 
ng without 
ability and 
d telephone 
ndings are 



Sound-powered sets find 
cipal application in mines, 
dent intercom systems. The 
and convenience of operati 
batteries and the service reli 
ruggedness of the sound-powere 
when used in adverse surrou 
points in their favor. 

2.2.1c Paging Telephones 



The most common type of communica- 
tion system used in underground mines is 
a paging telephone system. These phones 
are sometimes referred to as squawk 
phones or squawk boxes because of the 
harsh sound of the speaker. Each phone 
in the system is usually connected to a 
twisted pair cable in party line fashion. 
Each pager phone has internal batteries 
that power audio amplifiers to boost sig- 
nal level for normal communication. A 
paging amplifier allows each phone to 
broadcast a page call on a loudspeaker 
that is also housed within each phone 
(fig. 2-2). 

The paging telephone has gained 
widespread use for two reasons. First, 
it permits persons and station areas to 
be paged by name and thus does not 
require the miners to learn ringing codes 
or telephone numbers. Second, the use of 
page amplifiers in each phone makes the 



12 



T mSIEU PAm PHOHl Lwe 



I f t t I 



m 



TMMswrrcR- 



I l*^SS TO 
r TiLK 
SWITCH 



UTTtirr 



"1 




t 



FIGURE 2-2. • Generalized two-wire pager phone. 

system less affected by poor line splices 
and induced noise. The loudspeaker also 
yields a higher sound level at the 
receiver, which is important in the 
vicinity of noisy machinery. The primary 
disadvantage of paging telephone systems 
is that the telephone line must be used 
in a party line arrangement. This pre- 
vents simultaneous conversations in the 
system and reduces its usefulness for 
discussing maintenance problems or other 
uses which can tie up the system for long 
periods of time. 

In a large mine there may be 30 to 
40 phones on a single twisted pair cable. 
However, as the mine develops and the 
miles of twisted wire pair increase, a 
limit is reached. The limiting factor is 
the power .available to signal the paging 
amplifier in other phones to turn on. 
The application of an electronic switch, 
in place of the low-voltage "page relay," 
has extended the operating range of newer 
paging phones. 

A schematic of a generalized two- 
wire pager phone is shown in figure 2-2. 
When the ("Page") switch on the page 
phone is pressed, a dc voltage from the 
battery is placed on the line. All tele- 
phones connected to the line are ener- 
gized through their "page relay," and the 



paging amplifier at each station is 
turned on. The person making the call 
then squeezes the press-to-talk button on 
his handset and makes an announcement. 
The handset signal is amplified and then 
transmitted over the two-wire network to 
all other phones. After completing the 
page, the caller releases the "Page" 
switch. The individual paged can respond 
by squeezing the press-to-talk button and 
talking into the handset. Because the 
two-wire line is common to all phones, 
any conversation on the party line may be 
heard at all stations. 

Most of the pager phones that are 
available are directly interchangeable 
within a system or can easily be modified 
to be interchangeable. Two major areas 
of difference among available equipment 
are the battery voltage used for the 
page-call function, and the character- 
istics of the internal relay that 
responds to the dc page signal. The two 
choices of battery voltage are presently 
12 volts and 24 volts. The 24-volt sys- 
tem was the standard for several years. 
Most installations also used electro- 
mechanical relays to "switch in" the pag- 
ing amplifiers. 

In recent years, new designs using 
solid state switching circuits that oper- 
ate at 12 volts have become popular. 
Besides being more reliable, solid state 
systems have a high impedance and thus 
present a minimum load to the paging 
circuit. In a large complex multiphone 
system, with several miles of intercon- 
necting cable, the minimum loading caused 
by these phones means that more phones 
can be installed. 

Options included by the manufac- 
turers of some paging phones include a 
battery-test button, an improve-hearing 
button, and a flashing light to help 
attract attention in noisy areas. The 
battery-test button, when pressed, lights 
a lamp on the front panel to indicate 
that the battery is in good condition. 
When the improve-hearing button is 
pressed, the gain of the receiver ampli- 
fier in the handset is increased. To 



13 



assist the loudspeaker in attracting a 
miner's attention, the flashing light is 
turned on when the page is initiated. 

2. 2. Id Dial-and-Page Telephones 

The use of normal surface-type tele- 
phones in underground mines has two 
disadvantages: The potential hazard, in 
a methane atmosphere, from the 120-volt 
20-hertz bell-ringing voltage; and the 
inability to locate a person who is not 
in his inmiediate work area. Surface-type 
telephones also have two advantages, how- 
ever: The selective call feature and the 
multiple private lines. 

A unique system combines the dial 
telephone with the page phone. A surface 
interface is provided to isolate the 
potentially hazardous voltages from the 
underground line, and a converter changes 
ring voltage into the low-voltage direct 
current required to turn on the paging 
speaker in the dial-selected pager phone. 
The handset switch elin?inates the need 
for a hook switch. Pressing the handset 
switch accomplishes all functions nor- 
mally accomplished by lifting the handset 
of a conventional phone from the cradle. 
When a call is dialed, the interface mod- 
ifies signaling to interface another 
underground phone or a conventional tele- 
phone on the surface. 

Figure 2-3 is a diagram of a dial- 
and-page phone. The telephone consists 
of a handset, containing a transmitter in 
the mouthpiece and a receiver in the ear- 
piece, and a main housing. The main 
housing contains a speech dialer network, 
which isolates the outgoing and incoming 
signals, a pulsing switch that is actu- 
ated by the rotary dial to signal a num- 
ber to the surface interface, and the 
paging speaker. The speech network fil- 
ters noise and processes the talker's 
voice. A common-page button permits 
paging all phones, as is required when 
searching for a roving miner or for mak- 
ing a general announcement. 

Automatic dial systems are used at 
several mines that operate their own Pri- 
vate Automatic Branch Exchange (PABX) for 




OUTPUT 
TERMINALS 
TO SURFACE 
INTERFACE 



FIGURE 2-3. - Mine dial phone. 

both inside and outside telephone ser- 
vice. Dial systems provide for many 
simultaneous conversations, but do need 
to use some form of signal multiplexing 
or multiconductor cables. These problems 
can be minimized by using multiconductor 
cables only in the main haulageway where 
few roof falls or line breaks are likely 
to occur and by taking conventional two- 
wire cable up to each section. However, 
this means all telephones in one area 
must be on a party line. 

2.2.2 Distribution Systems 

The distribution system for wired 
phones is the equivalent of the string 
between the two tin cans described in the 
introduction. This is the propagation 
medium that carries the voice signal. 
For wired phones, the distribution system 
may be single pair, multipair, or a mul- 
tiplex system. 

All wired systems used in mines are 
inherently unreliable. That is, if a 
telephone line is broken or shorted by a 
roof fall, for example, all telephones 
beyond that point are severed from com- 
munication to the outside. If the line 



14 



is shorted, communications in the entire 
system may be severely affected or lost 
con5)letely . 

2.2.2a Single Pair 

A single pair of wires is the mini- 
mum requirement for wired comminications. 
A single twisted pair is used for magneto 
phones, paging phones, and intercom 
phones, connected in party line fash- 
ion. Single-pair wires are generally 
14 AWG (0.06408-inch diameter). This 
relatively heavy gage wire is used in 
order to minimize signal loss over long 
distance. 

The signal loss per mile, or attenu- 
ation as it is called, is dependent upon 
the frequency of the signal and the size 
of wire gage as shown in table 2-1. This 
table shows that as wire size increases 
and gage decreases, the signal loss 
(attenuation) decreases. 

Each attenuation of 3 dB means that 
the signal power has decreased by one- 
half. For example, if the output of a 
communication device was 4 mW, the -3 dB 
attenuated value of this signal is one- 
half the output, or 2 mW. A 6-dB attenu- 
ation of a 4-mW signal results in a 1-mW 
(4 X 1/2 X 1/2) signal level. 




FIGURE 2"4. " Figure-8 multipair cable. 
2.2.2b Figure-8 Multipair Cable 

Figure-8 aerial distribution wire is 
shown in figure 2-4. In cross section, 
the wire looks like a figure-8 with a 
steel support wire, called the messenger, 
in the top half and a core of twisted 
pairs in the lower half. The outer 
jacket covering the messenger cable and 
twisted pair bundle is highly resistant 
to abrasion, moisture, and environmental 
stress cracking. Each wire is solid, 
commercially pure, annealed copper. One 
conductor of each pair is usually color 
coded by a solid body color with a con- 
trasting spiraling color strip. The 
other conductor has the complementary 
color combination. Figure-8 cable is 
recommended for mine applications be- 
cause the messenger cable adds consider- 
able strength to the cable and the in- 
stallation is similar to that of trolley 
wire. 



TABLE 2-1. - Cable attenuations 





Wire size 


Frequency 


13 


gage 


16 gage 


19 gage 


(kHz) 


Decibels 


Percent of 


Decibels 


Percent of 


Decibels 


Percent of 




per mile 


signal power 
remaining 


per mile 


signal power 
remaining 


per mile 


signal power 
remaining 


10 


0.80 


83 


1.32 


74 


2.43 


57 


20 


1.04 


79 


1.55 


70 


2.77 


53 


30 


1.27 


75 


1.78 


66 


3.02 


50 


50 


1.75 


67 


2.24 


60 


3.53 


44 


100 


2.72 


54 


3.31 


47 


4.80 


33 


150 


3.60 


44 


4.27 


37 


6.00 


25 



15 



2.2.2c Coaxial Cable 

Coaxial cable consists of an inner 
conductor and an outer conductor, as 
shown in figure 2-5. This type of cable 
has two main advantages over twisted pair 
for transmission. First, the coax usu- 
ally has lower attenuation. Second, the 
shield over the central conductor keeps 
the electrostatic and electromagnetic 
fields contained within the coax, thereby 
minimizing crosstalk and interference 
problems. 

2. 2. 2d Multiplex Systems 

The term "multiplex" is applied to 
any system in which a single wire or wire 
pair is used for the transmission of more 
than one simultaneous signal. In this 
type of system, a means must be provided 
for inserting the individual signals onto 
the conmion transmission line and then 
separating these individual signals at 
the output of the line. There are two 
principal methods of multiplexing sig- 
nals. One is based on frequency transla- 
tions and is called frequency-division 
multiplexing, or FDM. The other is based 
on time-sharing the transmission line and 
is called time-division multiplexing, or 
TDM. 

2.2.2d.i Frequency Division Multiplexing 

Frequency division multiplexing is a 
process by which two or more signals are 
sent over the same line by transmitting 
each signal at a different frequency. 
The FDM concept can be illustrated by 
considering how the commercial radio 
broadcasting systems operate. Each radio 
station transmits at a specific frequency 
that has been assigned by the Federal 
Communications Commission (FCC). The 
signal from each radio station is trans- 
mitted by a common path (the atmosphere) , 
with many stations being on the air at 
the same time. To receive a particular 
station, a person merely tunes his radio 
(receiver) to the frequency of the 
desired radio station. 

This same principle, transmitting 
more than one voice signal, each at a 
different frequency, over a common path 



(in this case a single pair of wires) , 
can be employed in underground communica- 
tion systems. 

Figure 2-6 illustrates the FDM con- 
cept. At the transmitting terminal, each 
of the voice channels (CH 1 through CH n) 
is applied to a modulator. Each modu- 
lator shifts that voice signal to an 
assigned frequency (f^ through fn) and 
transmits the resulting signal over the 
common line to the receiving terminal. 

At the receiving terminal, a bank of 
filters separates the signals according 
to frequency. Individual demodulators 
recover the original voice signal. Note 
that the multiplexing system shown in 
figure 2-6 operates in only one direc- 
tion. Most communication systems require 
two-way voice transmission. This is 
accomplished by a complete duplication of 



INSULATION 



OUTER JACKET 




CENTER CONDUCTOR 



OUTER CONDUCTOR 



FIGURE 2-5. - Coaxial cable, cross-sectional view. 



TRANSMITTING TERMINAL 



RECEIVING TERMINAL 



— » FILTER — ij 



COMMON LINE 
FDM SIGNAL 
If, *U*U* 



FILTER 


J^ 


DEM 








FILTER 


i 


DEM 





FIGURE 2-6.- Frequency division multiplex (FDM)t 



SINGLE WIRE PAIR OR COAX 



y jL A 



MODEM *■] 



XX 



e 



xr 



© 



FIGURE 2-7, - Multiplex phone system. 



16 



multiplexing facilities, with the compon- 
ents in reverse order and with the signal 
waves traveling from right to left. Each 
terminal has a transmitting modulator and 
a receiving filter and demodulator com- 
bined to form a "modem." 

A block diagram of the multiplexing 
principle is shown in figure 2-7. Each 
phone is connected to a subscriber termi- 
nal unit or modem. The transmission line 
is a twisted pair or coaxial cable whose 
gage depends on the system size. Repeat- 
ers can be inserted in the line to com- 
pensate for attenuation. 

2.2.2d.ii Time Division Multiplexing 

Time division multiplexing is a 
process by which two or more signals are 
transmitted over the same line by allo- 
cating a different time interval for the 
transmission of each signal. The time 
available is divided up into small slots, 
and each of these is occupied by a piece 
of one of the signals to be sent. The 
multiplexing equipment scans the input 
signals in a sequential round-robin 
fashion so that only one signal occupies 
the TDM line at any one time. 

The basic concept, showing two sig- 
nals (A and B) being time division 
multiplexed together, is illustrated in 
figure 2-8. During time slot 1, signal B 




12 3 4 5 6 7 



FIGURE 2-8. - Time multiplexing two signals. 



is connected to the TDM line. During 
time slot 2, signal B is removed and sig- 
nal A is placed on the TEM line. This 
process is repeated with each signal 
alternately occupying a time slot on the 
TDM line. The bottom waveform in 
figure 2-8 shows the resulting time divi- 
sion multiplexed signal. The TDM signal 
consists of signal A during the even- 
numbered time slots and signal B during 
the odd-numbered time slots. To separate 
the signals, when they are received at 
the other end of the TDM line, a demulti- 
plexer must be used. The demultiplexer 
is similar to a multiplexer except that 
the input and output are reversed. The 
demultiplexer reconstructs the original 
signals (A and B) from the multiplexed 
signal. 

In order to illustrate the TDM con- 
cept, the preceding discussion considered 
multiplexing only two signals together. 
This concept can be extended to multiplex 
many signals together onto a single TDM 
line. For instance, if eight different 
signals are to be multiplexed, then each 
signal would be placed onto the TDM line 
each eighth time slot. Figures 2-9 and 
2-10 show eight switches at one location 
and eight lamps at another which must be 
controlled by those switches. 

The system shown in figure 2-9 is 
simple and easy to understand; however, 
eight separate wires must be strung 
between the switches and the lamps. This 
can be quite costly and impractical when 
the switches and lamps are separated by 
large distances. Figure 2-10 shows how 
each lamp can still be controlled by its 
associated switch using a single wire 
(TDM line) between the switches and 
lamps. With the wiper on each of the 
multipole switches synchronized, this 
system would sample the status of the 
first input switch (SI) at the trans- 
mitting end and communicate that informa- 
tion to the receiving end. At the next 
interval of time, both multipole switches 
would step to position 2. Control of the 
second lamp would be accomplished by sam- 
pling the status of the S2 input switch 
at the transmitting end. At the next 
time interval, both scanner switches 
would step to position 3 and a similar 



17 



INDICATOR 
LAMPS 



i * o— o— 



SI 
S2 



S3 



'Si 



S6 



POWER I , 

SUPPLY ^ 



FIGURE 2=9,- Nomultiplexing (eight wiresrequired). 



INDICATOR 
LAMPS 




FIGURE 2-10, - Time division multiplexing (one wire), 

control action would result for the third 
lamp. After all eight positions had been 
scanned, the scanner switch would return 
to position 1 and start the sequence over 
again. If the scanning were fast enough 
the lights would appear to glow steady 
and not blink. In a like manner, if this 
were a voice system, it would not sound 
chopped. 

2.2.3 Telephone Exchanges 

The function of any telephone 
exchange is to connect a calling phone to 
a called phone. The earliest method of 
connection was the manual switchboard 



located within a private branch exchange 
(PBX). As the number of phones in use 
increased, the size and complexity of 
manual switchboards also increased. This 
led to the development of an automatic 
switching system called a private auto- 
matic branch exchange (PABX). Since tel- 
ephone lines are currently used as data 
links for computers, teletypwriters , and 
various other data equipment, faster, 
more reliable exchanges were needed. To 
fill this need, the computer is presently 
used to control large solid state 
exchanges and perform many other central 
office (CO) functions. 



_L 2.2.3a Manual Switchboard 



The earliest switchboards allowed 
the operator to manually patch two cir- 
cuits together. When the central office 
received ringing current, the operator 
inserted the answering plug into the jack 
of the caller. After verbally receiving 
the called number, the operator inserted 
the calling plug of the same circuit into 
the called party's jack and applied ring- 
ing voltage on the ringer of the called 
party. After the conversation has been 
concluded, both parties would ring off, 
informing the operator that the plugs 
could be disconnected. 

2.2.3b Private Automatic Branch Exchange 

The private automatic branch 
exchange performs the same end function 
as the operator at the old manual switch- 
board. It makes a connection between the 
phone line of a caller to the line of the 
phone being called. Connections through 
PABX's can be made using electro- 
mechanical devices (rotary switches, 
relays, etc.) or solid state electronic 
circuits. 

In rotary dial phone system, the 
phone loop, which includes the two speech 
wires and the telephone set, is momentar- 
ily interrupted by the dial switch as the 
dial runs down. The number of loop 
interruption electrical pulses thus gen- 
erated corresponds to the number dialed. 
In pushbutton-type phone systems, the 
numerical information (each digit dialed) 



18 



is transmitted to the switching equipment 
in the form of coded frequency or voltage 
signals. In either case, the PABX 
switching equipment must receive and 
decode the "number dialed" signals from 
the calling phone to determine what phone 
is being called. 

2.2.3c Computer-Controlled Switches 

Modern solid state exchanges under 
computer control can efficiently handle 
thousands of phone lines. In addition to 
being faster and more reliable, computer- 
controlled equipment occupies only a 
fraction of the space required by systems 
using mechanical relays or rotary 
switches. 

Alterations to these systems no 
longer require a lot of time and hardware 
since the switching is under control of a 
computer program. Changes such as adding 
phones, deleting phones, changing a phone 
number, etc. , can be made by simply 
changing the program. The computer also 
can keep track of billing functions and 
special events. Special features, such 
as conference calls, automatic call for- 
warding, and abbreviated dialing, can be 
incorporated into computer-controlled 
exchanges by making changes to the com- 
puter program. 

System maintenance can also be han- 
dled by computer program. Instead of 
many labor hours spent in attempting to 
locate and correct a fault, the computer 
can cycle through all parts of the system 
and locate a trouble spot within a matter 
of minutes. Because computer operations 
are very fast, they can be performing 
maintenance functions even while handling 
the switching function for thousands of 
calls. 

2.3 Radio Systems 

Radio systems do not depend on a 
wire connection between transmitter and 
receiver. There are many types of sys- 
tems in this category: one-way voice, 
one-way signal, and two-way voice. In 
one-way operations, the transmitter sends 
a code or voice signal to the receiver. 



Two-way voice utilizes a device called a 
transceiver (combined form of transmitter 
and receiver). 

2.3.1 General Radio System Theory 

Voice frequency (VF) signals could 
be transmitted directly from one antenna 
to another; however, because of the low 
frequencies (and therefore long wave- 
lengths) involved, the antennas required 
would be very large. For this reason, VF 
signals are combined with higher fre- 
quency RF (radio frequency) carrier sig- 
nals which can be effectively transmitted 
and received by antennas of reasonable 
size. The two primary methods of com- 
bining VF signals and RF carriers are 
amplitude modulation (AM) and frequency 
modulation (FM). 

2.3.1a Amplitude Modulation 

In amplitude modulation the height, 
or amplitude, of the RF carrier is made 
to vary with the VF signal. This prin- 
ciple is illustrated in figure 2-11. The 
top waveform shown in figure 2-11 is a 
typical VF signal. The middle waveform 
of figure 2-11 represents an RF carrier 
that will easily propagate between an- 
tennas of convenient size. The bottom 
waveform shows the result of "amplitude 
modulating" the RF carrier with the VF 
signal. This signal retains the basic 
shape of the original VF signal but will 
also easily propagate between antennas 
because it is being transmitted at the RF 
carrier frequency. The original voice 
signal is regenerated by demodulation 
circuits in the receiver. 

2.3.1b Frequency Modulation 

A voice signal may also be super- 
imposed on a carrier frequency through 
the use of frequency modulation tech- 
niques. In FM, the frequency of the RF 
carrier is made to vary at the VF signal 
rate. As the amplitude of the VF signal 
changes, the frequency of the RF carrier 
(instead of the amplitude) changes. 

Figure 2-12 shows the principles of 
frequency modulation. The voice signal 



19 




A A A A ^'' 

I I I I I CARRIER 




RF CARRIER 
AFTER 
AMPLITUDE 
MODULATION 




FIGURE 2=11. . Amplitude modulation (AM). 

Is shown by the top waveform and the RF 
carrier by the middle waveform. The bot- 
tom waveform shows the resultant 
frequency -modulated RF carrier. As the 
voice signal increases to its maximum 
value, the carrier frequency increases 
(the waves bunch up). As the voice sig- 
nal decreases to its minimum value, the 
carrier frequency decreases (the wave 
spreads out). The amount of carrier fre- 
quency change is referred to as frequency 
deviation or carrier deviation. The 
unmodulated carrier frequency is referred 
to as the center frequency. The amount 
of carrier deviation is proportional to 
the amplitude of the voice signal, with 
maximum carrier deviation occurring at 
the peaks of the voice signal. 




VF 
SIGNAL 



RF 
CARRIER 



RF CARRIER 
AFTER 
FREQUENCY 
MODULATION 



A 



[\ 



u u u 



(\ (\ 



\J \J 




(\ A 



u 




II A A A 

_ 




Hiu U U 


J \j \j \. 



FIGURE 2-12. = Frequency modulation (Ff> 



FM receivers are less susceptible to 
noise when receiving a signal of only 
moderate strength or when the background 
electromagnetic (EM) noise is almost as 
"loud" as the signal. This advantage of 
FM over AM can become an important con- 
sideration in underground mining opera- 
tions where electrical equipment is being 
operated and the amount of EM noise being 
generated is large. 

2.3.2 Distribution Systems 

For the most part, the distribution 
system or propagating medium for radio 
transmission is not hardwired but takes 
the form of electromagnetic (radio) waves 
in the air. (Radio waves at certain 
frequencies will also propagate directly 
through the earth. ) Electromagnetic 
energy (radio waves) in empty space 
travel at the speed of light. Because 
the speed of any traveling wave is its 



20 



wavelength times its frequency, we have a 
formula of propagation (fA = speed of 
light) where X is the wavelength and f is 
the frequency. \ and f are thus 
inversely proportional (as f increases, X 
decreases), as shown in figure 2-13. 
Commonly used units are X in meters and f 
in hertz (Hz) (cycles per second). If 
the wave is traveling through anything 
other than empty space, its speed is 
reduced depending upon the electrical 
properties of the medium through which it 
is passing. Radio waves are slowed down 
only slightly by the earth's atmosphere. 
In solid insulating materials the speed 
is generally much slower; for example, in 
distilled water (which is a good insu- 
lator) the waves travel only one-ninth as 
fast as they do in free space. In good 
conductors such as metals the speed is so 
low that opposing fields induced in the 
conductor by the wave almost cancel the 
wave itself. This is the reason why thin 
metal enclosures make good shields for 
electrical fields at radio frequencies. 

2.3.2a Antenna Theory 

In normal electronic circuits the 
physical size of a circuit is small com- 
pared with the wavelength of the fre- 
quencies being used. When this is the 
case, most of the electromagnetic energy 
stays in the circuit itself or is con- 
verted into heat. However, when the 
physical dimensions of wiring or compon- 
ents approach the size of the wavelength 
being used, some of the energy escapes by 
radiation in the form of electromagnetic, 
or radio, waves. Antennas can be con- 
sidered as special circuits intentionally 
designed so that a large part of the 
energy input to the antenna will be 
radiated as electromagnetic energy. 



Usually an antenna is a straight 
section of conductor, either a wire or 
hollow metal tubing, which is suspended 
in space. When a radio transmitter is 
connected to the antenna, rapidly varying 
electrical currents are set up in the 
antenna. These currents cause electro- 
magnetic waves to radiate from the 
antenna and travel through the atmosphere 
or other surrounding medium. When these 
waves strike another antenna they induce 
electrical currents in it similar to the 
current flowing in the transmitting 
antenna. These currents, although they 
may be very small if the antennas are far 
apart or if they are transmitting through 
the earth, can be amplified by electronic 
circuits (receivers) to reproduce the 
original signal. The range of radio dis- 
tribution systems can be extended by 
leaky feeder cable (special coaxial cable 
designed to allow radio waves to "leak" 
from the cable to the surrounding atmos- 
phere and/or radio repeater stations. 

2.3.2a.i Half-Wave Dipole Antenna 

The strength of the electromagnetic 
field radiated from an antenna is propor- 
tional to the amount of current flowing 
in the antenna. It is, therefore, desir- 
able to make the current as large as pos- 
sible. This can be accomplished by 
adjusting the length of the antenna so 
that it resonates at the operating 
frequency. 

If a straight wire, or antenna ele- 
ment, were to be suspended in space, the 
lowest frequency at which it would 
resonate has a wavelength of twice the 
length of the wire. When used to trans- 
mit or receive RF energy that has a wave- 
length of twice the length of the wire. 



I02 io3 10* lO' 10® 



FREQUENCY IN HERTZ 
lOB io9 lO'O 10" 10'2 lo" I 



VOICE FRCO 

(vr) 



RADIO FREO - 
IRFl 



1 I I I 

INFDA-RED ! I 



iMEAT-WfcVESl 



liV 



I ULTRA I 
l< VIOUT«J 
RAYSI . 



SUN^ rays' 

*REACH~^ 
EARTH I ' 



3»I0* 



3XI02 



3XI0'2 



WAVELENGTH IN METRES 



FIGURE 2-13. ■= Electromagnetic energy spectrum„ 



21 




FIGURE 2=14. - Half-wave antenna, voltage and 
current distribution. 



1/2 X 



FIGURE 2-15. - Dipole antenna. 

the wire is known as a half-wave antenna. 
The current and voltage distributions 
along such a wire are shown in figure 2- 
14. Such an antenna, when connected to a 
receiver as shown in figure 2-15, is 
called a dipole. 

2.3.2a.ii Quarter-Wave Antenna 

An antenna may also be a quarter 
wave in length. This is possible because 
of its connection to ground, which elec- 
trically acts as the other quarter- 
wavelength. Refer to figure 2-16. The 
ground plane reflects the quarter-wave 
antenna so it has electrical character- 
istics similar to those of a half -wave 
antenna. 

An antenna of this sort may be 
any odd multiple of a quarter-wavelength: 
1/4A, 3/4A, 5/4X, 7/4A, etc. These 
antennas are commonly used for low- and 
medium-frequency applications. If a 
quarter-wave whip antenna is installed on 
a vehicle, the vehicle becomes the ground 
plane. A modified quarter-wave antenna 
is commonly used for citizens' band (CB) 
radios on vehicles. 




FIGURE 2-16. - Quarter-Vi^ave antenna, 
voltage and current distribution. 

2.3.2a.iii Long-Wire Antenna 

A long-wire antenna is one that is 
long with respect to the wavelength of 
the incoming and outgoing signals. The 
length should be an integral number of 
half -wavelengths (2A, 2-1/2A., 3A, 3-1/2A, 
etc.) to radiate effectively. A 1/2X 
(dipole antenna) is said to operate 
on the fundamental frequency, A oper- 
ates on the second harmonic, 1-1/2A 
operates on the third harmonic, 2A oper- 
ates on the fourth harmonic, and so 
on. 

2.3.2a.iv Loop Antenna 

Loop antennas can be utilized for 
through-the-earth radio transmissions or 
as receiving antennas in direction- 
finding systems. These antennas can be 
composed of one or more turns of wire on 
a round or square form, or the loop can 
be established by simply laying the wire 
in a loop on the ground or floor of a 
mine tunnel. 



22 



2.3.2b Leaky Feeder Systems 

Figure 2-17 shows a cross-section 
view of a standard coaxial cable and the 
lateral variation of its associated 
fields. In such cables, the bulk of the 
radio frequency electromagnetic energy is 
transported down the cable between the 
center conductor and the shield. How- 
ever, the shields of most coaxial cables 
do not provide perfect containment of the 
internal electromagnetic fields or isola- 
tion from external fields. As shown in 
figure 2-17, a small fraction of the 
cable's internal field is leaked to the 
external space. External fields also 
leak into the cable in a similar manner. 

The leaky feeder system is based on 
the use of semiflexible cable with spe- 
cially designed shielding that has a 
greater coupling to the external space. 
Therefore, this cable easily leaks radi- 
ated signals and saturates the area 
around the cable with these signals. One 
type of leaky feeder cable is shown in 
figure 2-18. The cable has a solid cop- 
per shield in which holes have been ma- 
chined to increase the amount of leakage 
to and from external space. In large 
mines, repeaters may also be used to 
amplify and retransmit incoming and out- 
going signals to roving miners carrying 
portable radios. The spacing of these 
repeaters along the cable is governed 
primarily by the receiver sensitivity, 
the longitudinal attenuation rate of the 
cable, the coupling loss from the cable 
to the portable units, and the trans- 
mitter power. Since the portable unit's 
transmitter power is generally lower than 
that available for fixed repeater or base 
stations, the portable units se't the cov- 
erage limits for two-way communications. 

OUTER COVER- 



COAXIAL CABLE 



CENTER 
CONDUCTOR 



►^ H'* — SHIELD 

INTERNAL FIELD STRENGTH 

T 




STRENGTH OF 
SIGNAL FIELDS 



EXTERNAL FIELD STRENGTH 



DISTANCE FROM CABLE 

FIGURE 2-17. - Coaxial cable, field strength. 
2.3.2c. Waveguide Propagation 

A waveguide is a hollow conductor, 
through which electromagnetic waves 
(radio waves) may propagate. Such a 
waveguide may be made of copper (the 
ideal), or other materials, such as coal 
or shale (nonideal). Hence, a mine entry 
is a waveguide. In order for a wave to 
efficiently propagate in a rectangular 
waveguide (mine entry or haulageway), the 
wavelength must be equal to or less than 
two times the greater dimension (a or b 
of figure 2-19). Table 2-2 shows the 
frequency spectrum designations with 
their wavelength ranges. The dimension 
of "a" that would be common in under- 
ground communications is 3 meters. This 
limits the lowest frequency range of 
signals that will effectively propagate 
within mine tunnels to the upper VHF and 
the UHF range. A communication device 
such as a CB radio has no application 
since it operates at approximately 
27 MHz, a frequency which is too low. 



HOLES IN SHIELD 



SHIELD-, 

/ ^CENTER CONDUCTOR 




-INSULATION 

FIGURE 2-18. - Cutaway view of leaky feeder cable. 



23 



TABLE 2-2. - Frequency spectrum designation 



Abreviation 


Description 


Frequency 


Wavelength range 


VF 


Voice frequencies ........... 


300-3,000 Hz 


10&-10S m 


VLF 


Very low frequencies 

Low frequencies ............. 


3-30 kHz 


105-10^ m 


LF 


30-300 kHz 


10'+-103 m 


MF 


Medium frequencies •••••••••• 


300-3,000 kHz 


103-102 m 


HF 


High frequencies ............ 


3,000-30,000 kHz 

30-300 MHz 


100 -10 m 


VHF 


Very high frequencies 

Ultra high frequencies 

Super high frequencies 

Extremely high frequencies.. 


10 - 1 m 


UHF 


300-3,000 MHz 


1 - 0.1 m 


SHF 

EHF 


3,000-30,000 MHz 

30-300 GHz 


10 - 1 cm 
1 - 0.1 cm 



Other factors that influence wave- 
guide propagation are wall texture (the 
smoother the wall, the better the propa- 
gation) and tunnel straightness , and 
electrical properties of the roof, walls, 
and floor. 

2. 3. 2d. Repeaters 

Two general types of repeaters will 
be considered for application in the mine 
environment: Single-frequency (Fl-Fl) 




HAULAGEWAY (NON-IDEAL) 




COPPER WALLS (IDEALI 



repeaters and frequency translation 
(F1-F2) repeaters. In its simplest form 
a repeater consists of two basic ele- 
ments, a receiver unit and a transmitter 
unit, as shown in figure 2-20. 

2.3.2d.i Fl-Fl Repeater 

Single-frequency repeaters are used 
in most wired systems such as coaxial 
systems where the transmission energy is 
confined within the coax. These repeat- 
ers function as signal amplifiers. The 
attenuated input signal is detected, 
amplified, and retransmitted. Since the 
signals are confined to a separate coax, 
isolation between transmitter and 
receiver is maintained. 

Single-frequency repeaters are also 
used in some wireless repeater systems, 
but extreme caution must be used 
to prevent feedback between repeater 
transmitter and receiver. Isolation must 
be maintained between the transmitter 
output and receiver input to prevent the 
transmitted signal from being received 
and amplified by the same unit. This 



RECEIVE 
ANTENNA 
OR COAX 



\ '/ 



TRANS- 
MITTER 



TRANSMIT 
ANTENNA 
OR COAX 



FIGURE 2-19. - Waveguides. 



FIGURE 2-20. - Basic repeater block diagram. 



24 



feedback can cause an oscillation or 
squealing problem very similar to that 
caused by placing a microphone in front 
of its own speaker. Directional antennas 
can be used to minimize this feedback 
problem; however, the use of high -gain 
directional antennas is not considered 
practical in the mine environment owing 
to installation problems and their sus- 
ceptibility to mechanical damage. 

Another method of overcoming these 
problems is to use special repeaters that 
do not depend on directional antennas. 
In general terms , these units function 
by separating the transmit and receive 
signals with time division multiplex- 
ing. The repeaters transmit pulses 
of RF energy and receive between these 
pulses. In this type of system all re- 
peaters must be phase-locked with each 
other to synchronize the time division 
process. 

2.3.2d.ii F1-F2 Repeater 

F1-F2 repeaters receive signals from 
a portable unit on one frequency (Fl) and 
retransmit these signals on another fre- 
quency (F2) to another portable unit. 
The mobile radios transmit on Fl and 
receive on F2. In this mode, all infor- 
mation goes to the repeaters, then back 
to the portable units. Some portable 
units are also capable of transmitting on 
F2 and, therefore, are able to talk to 
one another without the repeaters on a 
local simplex basis. With these systems, 
the receive and transmit antennas at the 
repeater are often covering the same 
general frequency bands and they can be 
combined so that only one antenna is 
required. 

Thus far, repeaters have been dis- 
cussed only as a means to permit communi- 
cation over greater distances than would 
be possible using direct transmission 
between portable radios. However, the 
audio link between the transmitter and 
receiver in the repeaters allows radio 
access to and from other types of audio 
circuits, such as specialized paging con- 
soles or the telephone system. 



One possible system configuration 
which includes both a telephone link and 
talk-through capability is shown in 
figure 2-21. This configuration allows 
for two modes of communications: F1-F2 
would be used for a local mode, that is, 
miner to miner within the working section 
through the repeater; and the second mode 
could support communications between a 
miner located in a working section with 
a second miner located somewhere else in 
the mine. The audio link (fig. 2-21) 
between the receiver and transmitter of a 
repeater can be used to customize repeat- 
ers to fit a variety of applications. An 
audio wireline can also be used to link 
a number of repeaters together to pro- 
vide complete radio coverage of the mine 
on a party line basis as shown in 
figure 2-22. 

2.3.3 Through-the-Earth Radio 

VF (0.3- to 3-kHz) radio waves will 
penatrate, to some extent, directly 
through the earth. Although signal 
strength is greatly attenuated, experi- 
ments have shown that up to 1,000 feet 



Fl 

RECEIVE 




F2 
TRANSMIT 



r' 



REPEATER 



AUDIO LINK 



I I 



TRANS- 
MITTER 



PHONE LINE 
INTERFACE 



TELEPHONE 
WIRELINE 



FIGURE 2-21i - Repeater linked to telephone system. 



25 



v^i 



REPEATER 



V 



V 



FIGURE 2*22. - Minewide repeater system linked by 
audio pair. 



(305 meters) of 
penetrated. 



overburden may be 



The transmitter may be a simple gen- 
erator with a loop or grounded wire 
antenna. The receiver may be a loop an- 
tenna connected to a power amplifier with 
a set of earphones or a meter. When the 
transmitter is activated, it sets up a 
magnetic field directly through the earth 
(the overburden). 

These characteristics of through- 
the-earth radio can be utilized in emer- 
gency situations to detect, locate, and 
even communicate with miners trapped 
underground. Once the position of an 
underground transmitting antenna has been 
determined using direction-finding tech- 
niques, a loop antenna can be positioned 
on the surface directly above the under- 
ground position. The trapped miner has a 
method of pulsing his transmitter off and 
on, such that a coded message may be sent 
to the surface. A high-power transmitter 
attached to the surface loop may also be 
utilized to establish down-link voice 
communications with the underground 
location. 



2.3.4 Radio Pagers 

Radio pagers are usually small FM 
radio receivers. The simplest type of 
radio pager is a one-way signal detector 
or "beeper. " These devices emit an audi- 
ble tone and/or blink a small light on 
and off. The miner must then go to the 
nearest phone to receive the message. 

Another type of radio pager is a 
one-way voice pager. These devices are 
similar to the simple beeper-type pagers 
except that the caller can deliver a 
short verbal message to the person being 
paged. A common type of one-way voice 
pager sounds a tone to alert the miner 
that a message will follow, and then 
broadcasts the verbal message. A disad- 
vantage of these pagers is that, although 
it is possible to transmit instructions, 
such as "turn off the power to number 4 
left," it is not possible for the caller 
to know for sure that the instruction was 
even received, let alone carried out. 
For this reason, one-way voice pagers 
should not be used to instruct personnel 
to perform specific tasks that may affect 
safety. 

Some one-way voice, and even beeper, 
pager systems allow the caller to selec- 
tively page a specific section or an 
individual miner. The heart of these 
systems is an encoder, which translates 
the number of each pocket pager to a spe- 
cific frequency or code that activates 
only the designated pager. 

2.4 Carrier Current Systems 

Any underground wire or cable, when 
fed an RF signal, tends to distribute 
that signal throughout its length. Car- 
rier current systems utilize this fact to 
establish communication paths using 
existing mine wiring. The wire used may 
be ac or dc power lines, neutral lines 
such as the hoist rope, existing phone 
lines, or other wiring. 

Carrier current devices are basic- 
ally FM radio transceivers that transmit 



26 



and receive over existing mine wiring 
instead of using an antenna system. The 
LF (low-frequency) and MF (medium- 
frequency) RF ranges propagate best in 
carrier current systems. A common exam- 
ple of a carrier current system is the 
trolley carrier phone systems presently 
used in many mines using trolley or rail 
haulage. Another example is the shaft 
communication systems that utilize the 
hoist rope itself to establish communica- 
tions to and from the cage. The most 
modern system, based on MF, promises to 
be the most effective of all. 



TROLLEYWIRE 




MICROPHONE WITH 
PUSH-TO-TALK 
SWITCH 



FIGURE 2-23. - Trolley carrier phone, block 
diagram. 



2.4.1 Trolley Carrier Phone 

A simplified block diagram of a 
typical trolley carrier phone is shown in 
figure 2-23. As mentioned earlier, the 
basic elements of any carrier cur- 
rent phone are the FM receiver and 
transmitter. 

In a trolley carrier current phone 
system, the receiver and transmitter are 
connected to the trolley wire through a 
coupler capacitor. The coupler capacitor 
acts as a short circuit at the frequency 
of the FM voice signals, but as an open 
circuit to the trolley wire dc power 
voltage. The high voltage levels on the 
trolley wire are thus blocked from enter- 
ing the receiver and transmitter sections 
of the carrier phone, while the FM voice 
signals pass freely through the coupler 
capacitor. 

The FM transceiver shown in 
figure 2-23 contains a power supply that 
converts trolley wire high voltage down 
to low voltage levels to provide power to 
the carrier phone circuits. The power 
supply may also contain a battery for 
backup power in case power on the trolley 
wire is lost. Such a system operates 
in the push-to-talk, release-to-listen 
mode. 

2.4.2 Hoist Rope Radio 

Figure 2-24 shows a block diagram of 
a hoist rope carrier current system. The 
system consists of two signal couplers 
and two transceivers. Each unit is 




M, 



^ HOIST ROPE 



TRANSCEIVER 




COUPLER CABLI 



CAGE COUPLER 



FIGURE 2=24. '- Hoist radio hardware. 

of the push-to-talk, release-to-listen 
design. During transmission, the sending 
unit feeds its coupler with a frequency- 
modulated (FM) carrier. The coupler 
induces a signal in the hoist rope, which 
is then picked up by the coupler of the 
second unit. Both couplers are elec- 
trically identical, and each operates 
both as a transmitting and as a receiving 
element. Operation of the hoist radio is 
the same as for a trolley carrier phone, 
except that the hoist radio signal is 
inductively coupled to the propagation 
medium (hoist rope). Some hoist phones 



27 



are simply modified trolley carrier 
phones. Other hoist phone systems have 
been specifically designed for operation 
in a vertical shaft and usually provide 
better coverage. 

The transceivers of the hoist room 
and cage are identical, except for the 
battery required in the cage. The hoist 
room power supply provides the power for 
the surface equipment. Surface equipment 
also may include a boom-type microphone 
and a foot-actuated push-to-talk switch 
to facilitate hands-free operation. 

2.4.3 Medium Frequency (MF) Radio 

Although radio transmission on 
the surface of the earth is well under- 
stood, transmission in an underground 
environment generally is not. Complex 
interactions occur between the radio wave 
and the environment. Characteristics of 
the geology (stratified layering, bound- 
ary effects, conductivity, etc.) and the 
mine complex (entry dimension, conduc- 
tors, electromagnetic interferences, 
etc. ) had to be measured and understood 
before a practical mine radio system 
could be built. To this end, consider- 
able research has been directed. 

In a confined area such as a mine, 
radio waves can propagate useful dis- 
tances only if the environment has the 
necessary electrical and physical proper- 
ties. The "environment" takes into 
account the natural geology and manmade 
perturbations such as the mine complex 
itself. As an example, if the wavelength 
(X) of a radio wave is small con^sared 
with the entry dimensions, a waveguide 
mode of propagation is possible. Attenu- 
ation depends primarily upon the physical 
properties of the entry such as cross- 
sectional area, wall roughness, entry 
tilts, and obstacles in the propagation 
path. Secondary effects such as the 
dielectric constants and earth conduc- 
tivity also influence attenuation. 



Mine radio systems based upon this 
effect are available commercially. These 
are UHF systems operating around 450 MHz 
which provide useful but limited cover- 
age. In high coal (6.5 feet), line- 
of -sight ranges of 1,000 feet are often 
possible. Range is reduced severely in 
non-line-of -sight, such as when going 
around a coal pillar. In lower coal, or 
when obstacles exist in the propagation 
path, range is reduced even more. For 
this reason, conventional UHF radio sys- 
tems require an extensive network of 
leaky feeder transmission cables and 
repeaters to become useful. Even so, 
range from the cable is not usually in 
excess of 30 to 50 feet, and equipment 
cost is very high. Clearly another 
approach is desirable. 

An important contribution to under- 
ground radio communications was made by 
the Chamber of Mines of South Africa. As 
early as 1948, programs were in place to 
develop radio systems for deep mines, 
primarily gold mines. The result was 
that by 1973, an advanced 1-watt single 
sideband (SSB) portable radio system had 
been developed that apparently worked 
well. The Bureau of Mines procured sev- 
eral of these units for evaluation. Per- 
formance in U.S. coal mines was not sat- 
isfactory. There were several reasons 
for this. First, U.S. mines are highly 
electrified, producing considerable elec- 
tromagnetic interference (EMI) not nor- 
mally found in the South African mines, 
which completely desensitized SSB radios. 
Second, 1 watt was not enough power. 
U.S. mines are mostly room and pillar, 
which means that any radio system would 
have to have reasonable range from local 
conductors. Third, geological electrical 
parameters were less favorable in the 
United States. For these reasons, the 
South African system was not acceptable. 

The Bureau's approach was to first 
determine the actual propagation charac- 
teristics of MF in U.S. mines, and then 



28 



to relate the propagation to the under- 
ground environment such as the geology, 
entry size, existing conductors, and EMI. 
Several exhaustive in-mine measurement 
and analysis programs were conducted. 
These programs formed the foundation for 
the first true understanding of how MF 
propagates in a stratified medium of var- 
ious electrical parameters, which are 
often interlaced by manmade conducting 
structures (rails and power lines) and 
artificial voids (entryways). 





Loop 

transmitting 

antenna 



^c 



Coal 

or 

entry 



Figure 2-25 is a simplified geometry 
of an in-mine site that illustrates one 
of the most important findings of the 
measurement program, the "coal seam 
mode." For this mode to exist, the coal 
seam conductivity (Oc) must be several 
orders of magnitude less than that of the 
rock (Or). A loop antenna that is at 
least partially vertically oriented pro- 
duces a vertical electric field (E^) and 
a horizontal magnetic field (H(t>). In the 
rock, the fields diminish exponentially 
in the Z-direction. In the coal seam, 
the fields diminish exponentially at a 
rate determined by the attenuation con- 
stant (a), which in turn depends upon the 
electrical properties of the coal. An 
inverse square-root factor also exists 
because of spreading. The effect is that 
the wave propagates between the highly 
conducting rock layers bounding the lower 
conductivity coal seam. The fact that 
the coal may have entries and crosscuts 
is of minor consequence. 

In the presence of conductors, the 
picture changes considerably. In this 
case, the effects of these conductors can 
totally dominate the effects of the ge- 
ology. In general, the presence of con- 
ductors (rails, trolley lines, water 
pipes, air lines, phone lines) is always 
of advantage. 

MF can couple into, and reradiate 
from, continuous conductors in such a way 
that these conductors become not only the 
transmission lines, but also the antenna 
system, for the signals. The most favor- 
able frequency depends to some extent on 
the relationship between the geology and 



FIGURE 2-25. - The coal seam mode. 



Local mine wiring 



T/f, 
R/f, 



Global 
repeater 



T/f| or fz 
R/f, 



Base 
station 



T/f| or f2 

Vest 1— ' 
transceiver 



T/f| or f 
R/f, 



Vehicula 






Console 



FIGURE 2-26. = Global repeater concept, 

existing conductors. The frequency 
effects are quite broad. Anything from 
400 kHz to 800 kHz will usually be 
adequate. 

The MF system described here is 
based upon vehicular and personnel 
transceiver units, base stations, and 
repeaters. It applies prior fundamental 
research in the area of MF and utilizes 
the existing mine wiring network (power 
cables, trolley line, etc.) to achieve 
whole-mine coverage. The basic system 
configuration is shown in figures 2-26 
and 2-27. 

Figure 2-26 illustrates a minewide 
repeater-base station concept known as 
the global maintenance net. In this con- 
figuration, mobile units (persons using 
transceiver vests and/or vehicular trans- 
ceivers) can maintain local communica- 
tions by operating at frequency f i . The 
range of communications in this case is 
solely dependent on point-to-point radio 



29 



propagation, aided by parasitic coupling. 
A transmission on £2 causes repeater 
action to occur, permitting the two mo- 
bile units to be separated very large 
distances. To achieve this repeater 
action, it is only necessary for the 
transmitting unit to reach the repeater, 
either directly or by parasitic effects 
to the repeater line coupler. Communica- 
tions with a base station are also 
possible. 

Figure 2-27 illustrates a local 
repeater concept constituting a local 
cellular net. This local repeater is 
known as a "cellular repeater" because it 
illuminates a "cell" or area of the mine, 
such as a working section, only. The 
antenna for the cellular repeater is a 
dual wire loop attached to timbers or 
the rib. An interface to the mine tele- 
phone system permits communications "off 
section. " 

The system design is distributed in 
the sense that each net can be operated 
independently of the other. In practice, 
a net can be easily installed by coupling 
a base station (at the portal) to elec- 
trical conductors in the wire plant 
(phone lines, power lines, etc.). Mobile 
transceivers operating on the assigned 
net frequency communicate with each other 
and the base. Other nets use different 
frequencies and are installed in the same 
way. 

Two types of mobile transceivers 
have been developed for the system. 
These transceivers consist of vest units 
for individuals and vehicular units for 
rolling stock. Functionally the two are 
equivalent, differing only in power lev- 
els and physical configuration. These 
transceivers are shown in figures 2-28 
and 2-29. 

An important human factor problem 
was solved by the vest design. By plac- 
ing the radio circuit modules in pockets 
on the vest, the weight and bulk of the 
transceiver have been evenly distributed. 
The loop antenna is sewn into the back of 
the vest. The pockets are located where 
medical records show less frequency of 
injury. Sound is directed toward the 



Pager phone line (existing) 



T/f, 

R/f2 



Cellular 
repeater 



y. Transmitter 
y Receiver 



T/f| or fj 
R/fi 



T/f, or f. 



Vest 
transceiver 



J 



R/f, 



Vehicu 
transceiver 



-J 

lar —I 



FIGURE 2-27. - Cellular repeater concept. 




FIGURE 2=28.. - Vest transceiver. 



30 




FIGURE 2-29, - Vehicular transceivero 



ears from epaulet speakers. A hinged 
control head is conveniently located on 
the front. The design allows the miners 
to maneuver in tight quarters and perform 
normal mining tasks without catching the 
radio on obstructions. 

The vehicular unit can be conveni- 
ently placed on any mine vehicle. It is 
used in conjunction with a special loop 
antenna of advanced design that produces 
high magnetic moment. Mechanically, the 
antenna is enclosed in high strength 
lexan and is attached to the vehicle via 
special brackets. The lexan will not 
break even if severely flexed by impact. 

Besides the mobile transceivers dis- 
cussed above, the system also consists 
of fixed transceivers such as repeaters 



and base stations. (See figures 2-26 and 
2-27.) For proper system operation it 
is necessary that these fixed trans- 
ceivers have very efficient antennas so 
that the local wire plant can be prop- 
erly illuminated and signals on the wire 
plant are properly received. This 
efficiency is paramount for whole-mine 
coverage. 

The cellular repeaters use dual-loop 
antennas (for transmit and receiver) 
attached to the rib or posts in such a 
way that there is little danger of damage 
in normal mine activities. The trans- 
mit antenna produces a large magnetic 
moment that provides the signal for local 
cellular coverage, which is usually aided 
by parasitic coupling and reradiation 
effects. The receive antenna is similar. 



31 



The global repeater and base station 
use a newly designed RF line coupler (see 
fig. 2-30) that permits very efficient 
coupling to the mine wire plant. Like a 
current probe, the coupler can be easily 
clamped around local conductors. MF 
signal current flowing through the 
wire plant conductors produces a coupler 
output signal (Vq), which is applied 
to the input of the base station or 
repeater. 



The base station is intended to be 
placed where mine management finds it 
most advantageous, usually in the surface 
office complex or with the dispatcher. 
If desired, the base station can be 
controlled remotely via signal lines that 
allow the control console to be placed in 
a surface building for convenience, while 
the actual base transceiver is placed in 
the mine where it can more efficiently 
couple into the local wiring. Both the 
global repeater and the base station uti- 
lize the RF line coupler for maxi- 
mum efficiency. The cellular repeater 
is generally located in a working sec- 
tion. It enables the vest to operate as 
a mobile pager telephone by switchin; 
voice signals between the local pager 
telephone network and the vest. Vehic- 
ular radios can also operate in this 
mode. 



The system was developed around an 
interchangeable set of plug-in radio cir- 
cuit modules. The same receiver, synthe- 
sizer, and transmitter modules are used 
in the vehicular transceiver, base sta- 
tion, and repeaters. Servicing the 
equipment only requires troubleshooting 
to the board level. Since the equipment 
uses the same radio circuit modules, the 
performance specifications of all trans- 
ceivers are similar. The signaling used 
depends upon the specific network re- 
quirements. All receivers are designed 
with an adaptive noise-operated squelch 
network that allows every transceiver on 
the net to hear the same message (party 
line) . 




ACTUAL NETWORK 



FIGURE 2-30. - RF line coupler for base sta- 
tions and repeaters. 

subaudible tone is used in the vest 
transceiver to cause the cellular 
repeater to switch the message (page) to 
the pager telephone network. The re- 
peater includes both a noise-operated 
squelch and a subaudible tone squelch for 
use in telephone switching. Subaudible 
tone signaling is useful in identifying 
"stuck on" transmitters that can block 
the communications net. In-band signal- 
ing is useful in emergency situations. 



g 2.5 Hybrid Systems 



Each of the communications systems 
already discussed has some individual 
shortcomings. However, one system may 
complement another system to alleviate 
certain problem areas. A hybrid is an 
interconnection of two or more sub- 
systems , taking advantage of the benefits 
of each. 



2.5.1 Improvements in System Versatility 

As mines have grown and mining tech- 
nology has improved, needs have arisen 
for new and improved communication capa- 
bilities that cannot adequately be pro- 
vided by the traditional mine pager-type 
phones or trolley wire carrier phones. 
These needs include the following: 

1. Ability to communicate when the 
phone line or the trolley wire breaks. 



The transmitters are designed with 
both subaudible (100 Hz) and in- 
band (1,000 Hz) tone oscillators. A 



2. Ability to communicate with 
personnel not in the vicinity of a 
telephone. 



32 



3. Ability to communicate over pri- 
vate channels. 

4. Ability to deliver important 
messages during periods of heavy communi- 
cations traffic during emergencies. 

5. Ability to communicate with sur- 
face public phones. 

The following techniques are capable 
of satisfying the foregoing needs using 
hybrid systems: 

1. Underground phones with manual 
trunking or automatic switching can pro- 
vide privacy and an interconnection to 
the public telephone system on the sur- 
face. Also, a larger number of simul- 
taneous communications can take place 
with multipair or multiplexed phone 
systems . 

2. Low-frequency radio offers a 
means of paging and communicating 
directly using the mine structure within 
a working section, and through the mine 
overburden in times of emergency. 



interconnected with the public phone 
system on a selective or temporary basis. 
The intent of these systems is to provide 
greater emergency communication capabil- 
ity during off-hours. These systems 
enable a person at a mine pager phone 
to gain access to the public phone 
system, or permit access to the mine 
page or phone system from any public 
phone. 

Figure 2-31 shows one system in 
which the interconnect between public 
phone and mine phone is made automat- 
ically. In this type of system small 
hand-held tone-generators are required to 
activate the automatic interconnect at 
the mine office. 

If a person in the mine wants to 
reach a prearranged public telephone from 
his mine phone, he sends a tone via the 
tone generator and mine phone to a tele- 
phone interconnect unit of the surface. 
At this surface interconnect, the tone is 
detected and activates a relay which, in 
turn, automatically dials the preset tel- 
ephone number. 



3. Medium-frequency radio can be 
used with power cables, trolley wires, 
and roof bolts to provide haulageway and 
section paging throughout the mine to key 
mining personnel carrying pocket pagers. 

4. Very-high-frequency radio can be 
used with leaky feeder cable or coaxial 
cable and antennas, as a technique for 
guiding radio waves throughout the mine 
haulageways and entries. This technique 
can be used to provide whole-mine com- 
munications with hand-held radios carried 
by key mining personnel. 

5. Ultra-high-frequency radio can 
provide wireless communication between 
key roving miners carrying radios within 
a working section, without the aid of 
additional wiring. 

2.5.2 Dial Phone-Pager Phone Systems 

Interconnect devices are avilable 
that permit mine paging telephones to be 



r" 



TELEPHONE OFFICE | 
I J 



PUBLIC PHONE 





/ n ^ Tt 


CENTRAL 
EXCHANGE 


1 H^X 




1 



AUTO ANSWER 



MAIN OFFICE 



r' 



J 

MINE PHONE LINE 



LOUDSPEAKING PHONE 



o D 



I 

I SECTION I 

FIGURE 2-31.- Dial phone to pager phone 
hybrid. 



33 



When a person calls the "auto 
answer" telephone number from a public 
phone, the interconnect unit automat- 
ically answers the phone, and upon recep- 
tion of the audio tone from the outside 
party, connects the incoming call 
directly to the mine pager phone line, 
thereby enabling the calling person to 
page and talk to the desired person in 
the mine. 

Systems also exist where the inter- 
connection between the mine phone system 
and the public telephone system is made 
manually, such as by a person on the 
surface. 

2.6 Other Systems 

2.6.1 Seismic Systems 

A seismic system can be used for 
trapped miner location. If a miner 
strikes a roof bolt, floor, or rib of the 
mine with a heavy object, the vibrations 
travel through the earth to the surface 
and can be converted into electric 
signals by seismic transducers called 
geophones. These signals can be ampli- 
fied, filtered, and recorded. Because 
the shock waves reach individual geo- 
phones at different times, the seismic 
recordings can be analyzed and the loca- 
tion of the miner can be determined. 
Analysis of seismic signals is a highly 
specialized field and beyond the scope of 
this manual. This method requires the 
assistance of an individual trained in 
seismic methods. However, the seismic 
system is the only trapped-miner system 
presently in operation and accepted by 
MSHA. Every miner should have a sticker 
(fig. 2-32) affixed to the inside of his 
helmet that he can refer to if entrapment 
should ever occur. 

2.6.2 Stench System 

Stench is used primarily as an evac- 
uation warning. It should be introduced 
into the underground system at as many 
locations as possible, with the intake 



WHEN ESCAPE IS CUT OFF 



1. BARRICADE 

2. LISTEN for 

3 shots, then... 

3. SIGNAL by 

pounding hard 
10 times 

4. REST 15 minutes 
then REPEAT signal unti 

5. YOU HEAR 5 shots, which 
means you are located fS 
and help is on the way. Is 



(■|M|LI^!J21^ 




FIGURE 2-32. - MSHAsignaling sticker. 

air and the compressed air as priority. 
Wherever miners may be in the mine, 
driven air is required and eventually the 
driven air and stench will arrive at 
their location. Stench may be any 
clearly distinguishable odor. 

The delay time in a stench warning 
system is one of its most important 
drawbacks. Another very important 
negative point is that stench warning 
cannot inform the miner what has hap- 
pened, where it has happened, or what he 
should do. Many times this type of in- 
formation can be worse than no informa- 
tion at all. 

2.6.3 Hoist Bell Signaling 

Much of the communication between 
the various levels of a mine and the 
hoist room consists of hoist bell signal- 
ing. This is a one-way communication 
system by which miners can request a cage 
and/or desired level. 

In the hoist room, there is a power 
source for the system and a buzzer. Each 
shaft station has a buzzer and a pull 
bottle. The bottle, when pulled, closes 
a switch that sounds the buzzer in 
the hoist room and at all other levels 
(fig. 2-33). The number of times the 
bottle is pulled corresponds to a command 
code. 



34 



TWISTED 
PAIR 



TJimu, 



SWITCHED 
LINE 




HOISTROOM 
BUZZER 



'///////////////////, 



/JP 



POWER 
SOURCE 



110 VAC LINE 




{^ 



PULL 
BOTTLE 



FIGURE 2-33. » Hoist bell operation. 

2.6.4 Visual Pagers 

One of the most common type of mine 
communication devices is the pager phone. 
However, with these systems, many calls 
are lost because the phone is too far 
from a working face or the page is 
not heard owing to high ambient noise. 
Visual pager systems are being developed 
that may eventually alleviate this 
problem. 

The most common type of visual pager 
system consists of strobe lights located 
at strategic locations in the mine. 
These lights are controlled by a dis- 
patcher who can set or reset them as 
required. They are usually used in con- 
junction with pager phones. Some modern 
multichannel phones have a light on the 
face of the phone that provides the vis- 
ual paging function. 

2.7 Summary 

There are three basic types of com- 
munication systems used in underground 
mines: wired phone systems, radio sys- 
tem, and carrier current systems. Wired 



phone systems include common dial 
telephones, pager phones, dial-type pager 
phones, magneto phones, intercoms, and 
sound-powered phones. These may be con- 
nected in party line fashion using a two- 
wire pair, or in selective calling 
fashion using multipair or multiplex 
techniques. Wired phone systems may have 
no switchboard (party line) for small 
systems, a manual switchboard, an auto- 
matic exchange, or a more sophisticated 
computer-controlled switch. A major dis- 
advantage of any wired phone system is 
that a roof fall could disrupt communica- 
tions between miners and the surface. 

Radio systems include all wireless 
communication systems. Coverage is lim- 
ited in radio systems because of poor 
propagation of radio waves underground. 
Voice frequency ranges can be used 
for through-the-earth radio. Ultra-high- 
frequency radio can be used when it is 
combined with leaky feeder cables, anten- 
nas, and repeaters to extend coverage. 
Personal radio pagers can be used to sum- 
mon an individual to a wired phone. 

Carrier current systems utilize 
existing mine wiring to propagate RF sig- 
nals. An RF signal from a carrier phone 
is induced onto a cable and transmitted 
throughout the length of that cable. A 
transceiver, inductive or capacitive 
coupled to the carrier cable, receives 
the RF signal, strips off the carrier, 
and lets the electromagnetic voice signal 
activate a speaker or earphone. 

Modern MF radio systems are being 
developed that combine the best features 
of radio systems and carrier current sys- 
tems. In these systems, no physical con- 
tact to existing mine wiring is required. 

Since one system cannot usually sat- 
isfy all the communication requirements 
in a mine, interfaces have been developed 
to make hybrid systems. Hybrids (two or 
more systems interconnected) take advan- 
.tage of the beneficial qualities of one 
system to alleviate the deficiencies of 
another system. 



35 



BIBLIOGRAPHY 



1. Aldridge, M. D. Analysis of Com- 
munication Systems in Coal Mines. Bu- 
Mines OFR 72-73, May 1, 1973, 127 pp.; 
available from NTIS PB 225 862. 

2. Alton, J. E. A Dial-and-Page Tel- 
ephone System. Paper in Underground Mine 
Communications (in Four Parts). 1. Mine 
Telephone Systems. BuMines IC 8742, 
1977, pp. 18-26. 

3. Anderson, D. W. Stout, and H. E. 
Parkinson. Interconnecting New Communi- 
cations to Existing Systems. Paper in 
Mine Communications, Proceedings: Bureau 
of Mines Technology Transfer Seminar, 
Bruceton, Pa., March 21-22, 1973. Bu- 
Mines IC 8635, 1974, pp. 73-86. 

4. Bradburn, R. A., and J. D. 
Foulkes. Longv/all Mining Communications. 
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Communications. BuMines IC 8745, 1977, 
pp. 44-62. 

5. Bradburn, R. A. , and R. L. Lagace. 
UHF Section-to-Place Radio. Paper in 
Underground Mine Communications (in Four 
Parts). 4. Section-to-Place Communica- 
tions. BuMines IC 8745, 1977, pp. 3-30. 

6. Bradburn, R. A., and H. E. Parkin- 
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Machine Operators. Paper in Mine Com- 
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IC 8635, 1974, pp. 36-45. 

7. Chufo, R. L. , R. L. Lagace, and 
L. R. Wilson. Medium-Frequency Mine 
Wireless Radio. Paper in Underground 
Mine Communications (in Four Parts). 
4. Section-to-Place Communications. Bu- 
Mines IC 8745, 1977, pp. 65-72. 

8. Chufo, R. L., and R. G. Long. 
Pager-Phone Guidelines and Test Equip- 
ment. Paper in Underground Mine Communi- 
cations (in Four Parts). 2. Paging Sys- 
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9. Dobroski, H. , Jr. A Coaxial-Cable 
Telephone System. Paper in Underground 



Mine Communications (in Four Parts). 
1. Mine Telephone Systems. BuMines 
IC 8742, 1977, pp. 42-57. 



10. 



MCM-101 Call-Alert Pag- 



ing. Paper in Underground Mine Communi- 
cations (in Four Parts). 2. Paging Sys- 
tems. BuMines IC 8743, 1977, pp. 3-6. 



11. 



Radio Paging. 



Underground Mine Communications 
Parts). 2. Paging Systems. 
IC 8743, 1977, pp. 29-33. 



Paper in 

(in Four 

BuMines 



12. Dobroski, H. , Jr., S. J. Lipoff, 
and J. E. Trombly. Call-Alert Paging for 
Pager-Phone Systems, Paper in Under- 
ground Mine Communications (in Four 
Parts). 2. Paging Systems. BuMines 
IC 8743, 1977, pp. 24-28. 

13. Dobroski, H. , Jr., and J. E. 
Trombly. Visual Paging System. Paper in 
Underground Mine Communications (in Four 
Parts). 2. Paging Systems. BuMines 
IC 8743, 1977, pp. 16-23. 

14. Ginty, J. J., R. L. Lagace, and 
P. G. Martin. Applicability of State- 
of-the-Art Repeaters for Wireless Mine 
Communications. Arthur D. Little, Inc., 
Cambridge, Mass., July 1975. 

15. Lagace, R. L. Report Highlights 
of the Working Group on Electromagnetic 
Through-the-Earth Mine Communication 
Links. Proc. 2d WVU Conf. on Coal Mine 
Electrotechnology , Morgantown, W. Va. , 
June 12-14, 1974, pp. 12-1—12-18. 

16. Lagace, R. L. , W. G. Bender, 
J. D. Foulkes, and P. F. O'Brien. Appli- 
cability of Available Multiplex Carrier 
Equipment for Mine Telephone Systems. 
Arthur D. Little, Inc., Cambridge, Mass., 
June 1975. 

17. Lagace, R. L. , and H. E, Parkin- 
son. Two-Way Communications With Roving 
Miners. Paper in Mine Communications, 
March 21-22, 1973. BuMines IC 8635, 
1974, pp. 46-72. 



36 



18. Long, R. G. Technical Guidelines 
for Installation, Maintenance and Inspec- 
tion of Underground Telephone Systems. 
Arthur D. Little, Inc., Cambridge, Mass., 
June 1975. 

19. Murphy, J. N. , and H. E. Parkin- 
son. Underground Mine Communications. 
Proc. IEEE, V. 66, No. 1, January 1978, 
p. 26. 

20. Parkinson, H. E. Emergency/Non- 
Emergency Mine Communications. Paper in 
Mine Communications, March 21-22, 1973. 
BuMine IC 8635, 1974, pp. 3-16. 

21. Parkinson, H. E., and J. D. 
Foulkes. Conventional Telephone Equip- 
ment. Paper in Underground Mine Com- 
munications (in Four Parts). 1. Mine 



Telephone Systems, 
1977, pp. 3-17. 



BuMines IC 8742, 



22. Powell, J. A., and R. A. Watson. 
Seismic Detection of Trapped Miners Using 
In-Mine Geophones. BuMines RI 8158, 
1976, 8 pp. 

23. Sacks, H. K. Trapped-Miner Loca- 
tion and Communication Systems. Paper 
in Underground Mine Communications 
(in Four Parts). 4. Section-to-Place 
Communications. BuMines IC 8745, 1977, 
pp. 31-43. 

24. Spencer, R. H. , and H. E. Parkin- 
son. Roving Miner Paging. Paper in Mine 
Communications, March 21-22, 1973. Bu- 
Mines IC 8635, 1974, pp. 17-35. 



CHAPTER 3. —SOLUTIONS TO THE COMMUNICATION REQUIREMENTS 



37 



3.1 Introduction 

Three types of communication systems 
have become popular in solving communica- 
tion requirements underground: Loud- 
speaking pager phones, carrier current 
phones, and magneto ringing phones. 
Basically, all three are simply single- 
channel party line systems. Although 
these systems are quite reliable, the 
single channel creates a variety of prob- 
lems. For example — 

1. Since no call is confidential, 
messages are sometimes purposely made 
vague, especially if accidents or safety 
topics are being discussed. 



wired phone system by using a remote 
portable radio. 

3. Remote monitors that alert per- 
sonnel when there is a toxic or explosive 
gas buildup. 

4. Control interfaces that allow 
remote control of fans, pumps, or other 
devices. 

5. Transmitters and receivers that 
can serve as emergency links. 

6. Loopback that allows an alter- 
nate path of communications if the main 
path is cut. 



2. A potential user must literally 
"wait in line" until the channel becomes 
clear for his use; thus, when foremen 
have to wait to call in reports or supply 
requests, this single chan-nel creates a 
serious productivity bottleneck. 



This chapter focuses on equipment 
and methods to meet the special communi- 
cation needs of individuals in various 
places of the underground mine. The com- 
munication requirements can be broken 
down into four categories: 



3. In many large mines, there are 
independent branch lines that naist be 
tied together by a dispatcher, adding 
further delays to the system. 

To solve these problems , some mines 
have . installed other phone systems — 
mostly commercial dial phones in indus- 
trial enclosures that offer extra channel 
capacity and private line features. 
Others have installed a system that com- 
bines both dial- and page-phone features 
in a single unit. 



1 . The mine 
communication) . 



entrance (shaft 



2. Permanent and semipermanent lo- 
cations (shop areas, lunchroom, crusher 
stations, etc.). 

3. Mining areas (the room-and- 
pillar sections, longwall faces, block 
caving areas, etc.). 



4. Haulageways (tracked 
haulage, diesel, belt haulage). 



trolley 



Although these do represent improve- 
ments, they do not truly solve the over- 
all problems that face modern mines. 
Besides extra channels, a communica- 
tion system should have the following 
features to enhance productivity and 
safety: 

1. A means of paging a roving miner 
to alert him that he is wanted on the 
phone. 



Methods of implementing systems to 
meet the communication needs of these 
areas are described in sections 3.2 
through 3.5. 

Section 3.6 discusses methods of 
satisfying special communication require- 
ments that exist. Major topics in this 
category include communications with rov- 
ing personnel, the isolated miner, and 
motorman-to-snapper communications . 



2. Wireless-to-wired system inter- 
connects by which a miner can talk on the 



Emergency communication systems are 
described in section 3.7. Detecting and 



38 



locating the trapped miner, rescue team 
communications , and emergency warning 
systems are discussed. 

Although the methods of establishing 
communications throughout a mine are 
broken down and described in separate 
sections, as outlined above, it is impor- 
tant to realize that these systems should 
be tied together or interconnected in 
some way. The overall design plan must 
include provision for integrating the 
various communication subsystems together 
into a minewide system. Such a system, 
designed with the total mine operating 
plan in mind, will be the most effective. 
In a like manner, a judicious choice of 
monitored parameters in the underground 
environment and selected machinery will 
yield a cost savings in production and 
augment safety. Many man-hours and dol- 
lars can be saved by knowing conditions 
before they become a problem. Situations 
that could become disastrous can be pre- 
dicted and proper solutions implemented 
before the disaster occurs. Because 
proper environmental and machine monitor- 
ing and control is another key to safer 
and more productive underground mining, 
these factors should also be considered 
in the overall plan of any communication 
system. 

3.2 The Mine Entrance 

The entrances to underground mines 
are either vertical shafts, slope en- 
trances, or horizontal drifts. Slope and 
horizontal drift entrances can be consid- 
ered as a continuation of a haulageway 
and are treated in section 3.5. This 
section is devoted primarily to shaft 
communications. 

In the past, operators of single- 
level mines with overburdens less than 
1,200 feet have felt that communications 
between the top of the shaft, the bot- 
tom, the hoistroom, and possibly a com- 
munications center were adequate. Many 
mines in this category (which includes 
most underground coal mining operations) 
did not have the capability of two-way 
voice communication with personnel in the 
cage. 



One of the biggest reasons for this 
deficiency in communications to and from 
the cage has been that reliable equipment 
simply was not available for establishing 
this vital two-way voice communication 
link. This reason is no longer valid. 
Today, equipment is commercially availa- 
ble to implement effective two-way voice 
communication, even while the cage is 
moving, down to depths in excess of 
10,000 feet. 

A useful hoist-shaft communication 
system must satisfy the requirements for 
communication throughout the full travel 
of the cage, providing voice communica- 
tion between the cage and the hoistman, 
as well as to underground shaft stations. 
An effective system should also provide 
for shaft-inspection communication be- 
tween the inspector and the hoistman, and 
should have a slack-rope indication. For 
the modern, automated shaft, signals are 
also required to permit selection of 
level, enable interface with interlocks, 
and permit jogging for exact position at 
any level. 

The limited space within the cage 
places an operational restraint on equip- 
ment. Equipment must be small and should 
be located so that it cannot be damaged 
by any of the various uses of the cage 
such as transporting supplies and machin- 
ery. An additional microphone-speaker 
station may also be desired for multi- 
level cages or when several cages are 
joined together. 

A reliable hoist-shaft communication 
system should be considered as a vital 
part of any overall communication system. 
Shaft communication is especially impor- 
tant during or following an accident or 
disaster situation. Experience has 
taught that the hoist often becomes a 
bottleneck during rescue or evacuation 
operations, and good communication to and 
from the cage is essential. 

Traditionally, bell signals have 
been used between those requesting the 
cage and the hoistman, and until re- 
cently^ bell signaling was usually the 
only form of hoist-shaft communication. 



39 



Today, however, equipment is available 
that allows two-way voice communication 
between persons in the hoist cage and the 
hoistman or other locations at the shaft 
top or bottom. 

Presently available methods of im- 
plementing two-way voice communication 
with the hoist cage include trailing ca- 
ble systems, radio systems, and hoist 
rope carrier current systems. 

3.2.1 Bell Signaling Systems 

There was a time when bell signaling 
was the only form of communication 
between those requesting a cage and the 
hoist operator. Because of this, bell 
signaling systems have gained widespread 
acceptance and are used on many hoists. 

Figure 3-1 shows a simplified sche- 
matic diagram of a typical shaft buzzer 
signaling system. In the system de- 
picted, a single twisted pair wire is run 
down the mine shaft and "tapped off" at 
those shaft stations where signaling is 
required. Figure 3-1 shows a system with 
the twisted pair tapped at three levels 



UNDERGROUND 



SWITCHED LINE- 



^ 



HOISTROOM 
BUZZER 



77777777777777777777777777777, 



j_ « TWISTED PAIR 

— 110 VAC LINE 



^ 



^ 



PULL BOTTLE 



PULL BOTTLE 



^ 



-A'" 



PULL BOTTLE 



FIGURE 3-1. = Bell signaling syster 



(level A, level B, and level C). To sig- 
nal the hoist operator, a miner at any 
level pulls the pull bottle, causing the 
associated switch to close. This applies 
voltage to the buzzer at that level, and 
also to the hoistroom buzzer and all oth- 
er buzzers, through the switched line of 
the twisted pair. 

Bell signaling systems, although 
proven to be reliable, do have some 
severe shortcomings. First, there is no 
way to convey special messages to the 
hoistman. Special equipment or assist- 
ance or unusual cage movements cannot be 
requested unless a signaling code has 
been defined for that specific request. 
A second disadvantage, especially for 
mines with many shaft stations, is that 
the bell codes required can become quite 
long. Long or complicated bell codes are 
obviously more difficult to remember and 
can become a source of confusion, espe- 
cially during an emergency or disaster 
situation. During these critical periods 
of high emotional stress, mistakes are 
easy to make even when signaling codes 
are posted. Some mines with many levels 
and/or shaft stations have partially 
overcome this disadvantage by assigning 
an employee to the hoist cage. Because 
this miner, called a eager, is perma- 
nently assigned to act as the hoist cage 
operator, the bell signaling code has 
become "second nature" to him. 

Another disadvantage of the bell 
signaling system is that communication 
with the cage is impossible when it is 
between levels. This deficiency is 
especially crucial during shaft inspec- 
tion or repair. Some mines have par- 
tially overcome this problem by running a 
pull cord down the shaft. This cord is 
kept in a position next to the shaft tim- 
bers by staples and can be used for emer- 
gency stops and signaling between shaft 
stations. This system does provide some 
degree of emergency communication from 
the cage while it is between shaft sta- 
tions; however, operation of the system 
can be extremely dangerous since it 
requires the operator to reach out of the 
cage and grab a cord, which may be moving 
relative to the cage. 



40 



3.2.2 Trailing Cable Systems 

One method of establishing two-way 
voice communication with the hoist cage 
is by using a trailing cable. In this 
type of system. A communications cable 
is physically attached to the bottom of 
the cage and allowed to hang down the 
shaft below the cage. 

Figure 3-2 shows a typical trailing 
cable system. In the figure, three 
phones (one in the hoistroom and one at 
each of the two shaft stations) are con- 
nected by a phone line that has been 
strung down the shaft to a junction box 
located about halfway down the shaft. 
Connections are made within the junction 
box to the trailing cable which provides 
the link to the phone mounted in the 
cage. The trailing cable system can be 




///////// 



■/////////// 



'/ 



tied into the existing wired communica- 
tion system in the mine, or it can be 
implemented as an independent, shaft- 
only, communication system. 

In addition to the disadvantages 
associated with any wired communication 
system (normal cable maintenance and line 
breaks), the trailing cable system has 
limitations in terms of depth because of 
the amount of cable that can be trailed 
from the cage. 

3.2.3 Radio Systems 

Another approach to satisfy the 
voice communication requirements with 
personnel in the cage is by using two-way 
radio systems. Some recently installed 
radio systems are meeting the hoist com- 
munication requirements. One radio sys- 
tem currently being used at an iron ore 
mine is illustrated in figure 3-3. In 
this system, portable police-type 150-MHz 
FM radios were used in 19-foot-diameter 
shafts. The surface antenna is a dipole 
mounted on plywood bolted to the steel 
collar at the top of the shaft. A coax 
runs from the dipole to the hoist room, a 
distance of about 500 feet. In the cage, 
the radio and 12-volt battery are mounted 
in a plywood box. 

Results of studies indicate that the 
attenuation of radio signals increases 



//////// ///T/ 



rrjTTT 

FIGURE 3-2. - Trailing cable system. 




§ 



HOISTMAN'S RADIO- 
CONNECTED TO 
ANTENNA AT HEADFRAME 



RADIO SIGNAL 



FIGURE 3-3. - Hoist cage radio. 



41 



sharply as shaft size is decreased. For 
straight, unobstructed shafts with a 
diameter in the neighborhood of 12 feet, 
radios opetating in the frequency range 
of 500 to 1,000 MHz should provide commu- 
nication to a depth of approximately 
1,500 feet. For smaller diameter shafts, 
radio communications will only be possi- 
ble over shorter distances. 

3.2.4 Hoist Rope Carrier Current System 

A carrier current system using the 
hoist rope as the carrier has been devel- 
oped that provides reliable two-way voice 
communication between the cage (even 
while in motion) and the hoistman to cage 
depths in excess of 10,000 feet. The 
principle of operation of the hoist rope 
carrier current system is similar to that 
of the carrier current commonly used in 
trolley carrier phone systems. Both 
systems transmit and receive RF energy 
over a transmission line. In a trolley 
system, the transmission line is the 
trolley wire. In the hoist system, the 
carrier signal is transmitted on the 
hoist rope. Both systems utilize trans- 
mitters and receivers (transceivers) that 
communicate with each other by RF cur- 
rents superimposed on a cable. 



SHEAVE 



The principal diff 
trolley carrier system 
carrier system is the 
energy is transferred 
from, the transmission 
hoistman' s transceiver 
cannot be physically at 
touch) the hoist rope, 
of superimposing RF ene 
must be used. 



erence between the 

and the hoist rope 

way in which RF 

to, and received 

line. Because the 

at the headframe 

tached to (or even 

a different method 

rgy onto the rope 



The solution is to inductively 
couple the hoistman' s transceiver to the 
hoist rope. Figure 3-4 shows a block 
diagram of a hoist rope carrier current 
system. The system consists of two sig- 
nal couplers and two transceivers. Each 
transceiver is of the push-to-talk, 
release-to-listen design. During trans- 
mission, the sending transceiver feeds 
its coupler with an FM carrier. The 
coupler induces a signal into the hoist 
rope, which travels up and down the hoist 



HOIST 



SHEAVE 
COUPLER 




CAGE 



FIGURE 3-4. - Hoist rope carrier current system, 

rope and is picked up by the coupler at 
the other transceiver. Each coupler 
operates as both a transmitting and 
receiving element. The cage coupler is 
clamped to the hoist rope at a point just 
above the cage. The coupler for the sur- 
face transceiver should be permanently 
mounted below the sheave wheel and about 
6 inches from the rope. Coaxial cable 
should be used to connect each coupler to 
its transceiver. 

3.2.5 Hoist Signaling Summary 

The pull-bottle shaft bell has been 
the universally accepted means of cage 
signaling. More than 60% of the hoists, 
notably those in bedded deposit mines, 
have only one underground level; hence 
this type of signaling system is simple 
and effective. In multilevel mines, sig- 
naling codes become complex to the point 
where a full-time cageman may be required 
to control the cage during all man and 
equipment lifts. Depending on the size 



42 



and nature of the shaft, commercial radio 
equipment operating at 150 or 450 MHz can 
provide a voice link down to 2,000 or 
3,000 feet. 

For a very deep (2,500 feet or 
greater) or narrow shaft (less than 
10 feet in diameter) , comiminication sys- 
tems are available that inductively 
couple RF signals to the hoist cable. 
These carrier current systems provide 
two-way communication with the cage in 
even the smallest shafts down to depths 
in excess of 10,000 feet. 

3.3 Permanent and Semipermanent In-Mine 
Locations 

Looking only at the permanence of a 
telephone installation, phone locations 
can be divided into the following three 
categories: 

Permanent (life of the mine). 

Semipermanent (more than a year be- 
tween moves) . 



Frequently 
monthly). 



moved (weekly 



to 



Permanent locations include surface 
sites, the dispatcher's station, under- 
ground offices and shop areas, lunch- 
rooms, rail or belt heads, storage areas, 
the crusher operator, and along main 
haulageways. 

Semipermanent phones would be found 
mostly in the submains of a mine. After 
panels have been fully developed, most of 
the phones in the submain would be re- 
located to more active sections. One or 
two phones would remain for use by roving 
personnel. If a submain became part of 
the haulage system, in all likelihood 
more phones would remain in use to meet 
the operating practices of the mine. 

Frequently moved phones are pri- 
marily located near the working faces of 
the mine, typically in working sections 
off submains. These phones are moved 
with the section in order to maintain 
close communication with the dispatcher, 



maintenance, and management personnel. 
Communications equipment associated with 
advancing or frequently moved face areas 
is treated in section 3.4. 

The single-pair wired phone system 
is the communication system commonly 
employed to satisfy the requirements of 
permanent locations. Magneto phones were 
first used, but although many are still 
in use, they have been largely replaced 
by loudspeaking pager phones. 

In a few mines the conventional tel- 
ephone with a rotary dial and ringer 
(mounted in an explosion-proof housing) 
has been used. Systems using these dial 
phones are usually an extension of an 
aboveground private automatic branch ex- 
change (PABX) or a single-party indepen- 
dent system with a small switchboard. 
Recently multipair cable and even multi- 
plex systems have been used to intercon- 
nect phones and to connect individual 
phones to an aboveground PABX. 

3.3.1 Single-Pair Pager Phones 

Pager phones were specifically 
designed to meet the requirements of per- 
manent and semipermanent locations for 
underground mining operations. They dif- 
fer from a conventional telephone in that 
instead of a ringer, a loudspeaker is 
used in each phone to alert the person 
being called, and each phone has its own 
batteries for power instead of being cen- 
trally powered. In a single-pair instal- 
lation, the pager phones are inter- 
connected by a single twisted pair of 
wires and all phones are on a single 
party line. A 14- to 18-gage copper pair 
with a neoprene jacket is most often 
used. 

Figure 3-5 shows a hypothetical, 
moderate-sized room-and-pillar coal mine 
with an average working section size of 
300 feet by 400 feet, and an average 
panel size of 800 feet by 1,200 feet. 

The upper half of figure 3-5 shows 
the main haulageway, the operational sub- 
mains, and the working sections and 
illustrates how a single-pair pager phone 



43 



y^m^oL 



g SPLICE CASE 

9 FREQUENTLY MOVED PHONE 

SEMIPERMANENT PHONE 

PERMANENT PHONE 




SEMIPE.RMANENT _^-__j»r 

PHONE IN TTTj 

SUBMAIN ' 1 J-"^ 



oDDoa 



J — ^oa 



nnoQ 

QODCpo 
0)0 0000 

DCao 
DDQO 
qQOD 







EQUENTLY MOVED 
PHONE IS KEPT 
NEAR THE WORKING 
SECTION 



PERMANENT PHONE IN MAIN 



O QZDO C3C>D|C>O^Da OO O 9 O 



bP^DOQOQ 






FIGURE 3-5. • Single-pair pager phone installation. 

system would be wired. The solid black 
line represents the single pair. The 
black squares represent splicing points, 
and the circles represent the telephones. 
Telephones can be seen at the dis- 
patcher's office, in the underground shop 
area, and along the main haulageways; 
there, is one at each butt entry and one 
near each working section, and a twisted 
pair is shown running to the pager phone 
at each working section. When a panel is 
worked out, the phone at that butt entry 
may be removed and installed at another 
location. Usually, as shown in fig- 
ure 3-5, a few phones are left behind 
along the submains in worked-out sec- 
tions. These phones may be used peri- 
odically by roving maintenance personnel 
or inspectors. 

A mine is not static; its architec- 
ture or layout changes, but fortunately 
these changes are usually known well in 
advance so that cable selection and the 
phone line layout in the mine can be 
planned to accommodate future growth. As 
far as changes are concerned, the phones 
shown in the example of figure 3-5 fall 
into the three basic categories as 
follows: 



1. Telephones in the dispatcher's 
office, shop area, and main haulageway , 
and opposite each submain, would rarely, 
if ever, be moved (permanent). 

2. Telephones opposite each butt 
entry would remain in place for one year 
or so until more panels in the submain 
have been developed (semipermanent). 

3. Mine safety regulations require 
that a communication link must be estab- 
lished within 500 feet of the working 
face; hence, telephones at the working 
sections are required to be moved fre- 
quently (perhaps once a week). 

3.3.2 Multlpair Systems 

For a multlpair access installation 
(fig. 3-6) , planning for future mine 
growth becomes important. The figure 
shows an example of how a multlpair sys- 
tem may have grown in our hypothetical 
mine, which has four working sections (A, 
B, C, and D) . In this example, when the 
system was installed, working section A 
did not exist, so three-pair cable was 
used to give sections B, C, and D private 
lines. (The telephone at the working 
face is an extension of the butt entry 
phone, which may not be reasonable in low 
coal.) When section A came into opera- 
tion, either more cable had to be in- 
stalled or more telephones had to be con- 
verted into extensions without private 
lines. Figure 3-6 shows that six-pair 
cable was run along the main haulageway, 
so that at this stage of development, 
several telephones were forced to share a 
pair. 

Several telephones are extensions, 
and as long as that is a satisfactory 
condition, six-pair cable in the main 
haulageway Is sufficient. However, if 
the objective is to provide every tele- 
phone with its own pair (which really is 
the point of a multlpair dial access sys- 
tem) , additional cables have to be run 
down the main haulageway. The lesson, of 
course, is to keep future needs in mind 
when planning cable layouts, particularly 
in areas like main haulageways and main- 
tenance areas where telephone locations 
are unlikely to change for many years. 



44 



isssst^^s?^;:^^^ 




lA 2A 3A 






FIGURE 3-6. - Multipair installation. 



Figure 3-7 shows details of the 
multipair system; once again, the 
three categories of telephones — perma- 
nent, semipermanent, and frequently moved 
telephones — can be seen. 

3.3.3 Multiplexed Systems 

Various types of multiplexed systems 
can also be implemented to satisfy commu- 
nication requirements of permanent and 
semipermanent locations underground. 

One system using multiplex equipment 
and a small PABX has been installed in a 
deep, multilevel, metal mine in the West- 
ern United States. This system utilized 
an existing twisted pair already strung 
through the mine to establish two seven- 
channel private communication links. A 
simplified diagram of the system is shown 
in figure 3-8. 

The single twisted pair utilized by 
the systems extended from the surface, 
down shaft A to the 3,700-foot level, 
horizontally through a 5,000-f oot-long 




^^^^^^iii;^^; O SEMIPERMANENT TELEPHONE 

•^^ ® FHEQUENTLV MOVED TELEPHONES 
■ SPLICE CASE 

TELEPHONES 




FIGURE 3-7. . Detail of multipair. 



FIGURE 3.8„ - Multiplex system with PABX. 

drift to the underground headframe of 
shaft B, and then down shaft B to the 
5,600-foot level. An air-conditioned 
room was available in the shaft B area at 
the 3,700-foot level that met all envi- 
ronmental requirements of commercially 
available PABX systems. Additionally, 
this location was approximately centered 
with respect to the physical locations of 
the desired phones. The single twisted 
pair was opened at this point, thereby 
forming two independent wire pairs (one 
running back to the surface, the other 
running down shaft B). A carrier system 
was then installed on each pair, and 
these two independent carrier systems 
were then connected to the PABX line cir- 
cuits. This provided 14 private channels 
(lA through 7A and IB through 7B) for 
communication within the mine. This sys- 
tem (described in more detail in appendix 
A), not being intrinsically safe, is not 
suitable for use in coal mines. 

Presently, there is no intrinsically 
safe multiplexed telephone system de- 
signed for mine use that is commercially 



45 



available. However, the Bureau of Mines 
is developing such a system. This system 
provides eight full duplex channels, some 
of which can be dedicated to monitor and 
control functions. The system uses in- 
expensive twisted shielded pair and is 
not under control of any central switch- 
ing or control center. Because of this, 
the system will not be made inoperative 
because of a^ cable break or a central 
PABX failure. Other features include a 
message-leaving light on each phone, low- 
battery indicators, and compatibility 
with standard loudspeaking pager phone 
systems. 

3.4 Mining Area 

Safety regulations require that a 
communication link must be established 
within 500 feet of the working face. In 
coal mines a butt entry portable phone 
meets that requirement at the beginning 
of a panel's development, but a frequent- 
ly moved section phone must be installed 
once the face has moved 500 feet from the 
butt entry phone. Weekly movement of the 
section phone might be necessary to keep 
the section foreman within range. 



communication with other parts of the 
mine. 

Most existing mine communication 
systems stop at the last open crosscut of 
the section. Present mine communication 
systems are aimed at satisfying the need 
that the mine section foreman be able to 
communicate with the mine shift foreman. 
However, in some mines there may be addi- 
tional communication needs within the 
mine section. Needs that are not ade- 
quately met include communications be- 
tween the continuous miner operator and 
the shuttle buggy operators, between the 
shuttle buggy operators and the "gather- 
ing" locomotive operator, and between the 
general maintenance foreman and the sec- 
tion maintenance man repairing a machine. 
By satisfying these needs, both the safe- 
ty and efficiency of mining operations 
can be improved. The existing power 
trailing cables to the face machine pro- 
vide one means to achieve these communi- 
cations capabilities in a reliable and 
economic manner. Another method of es- 
tablishing voice communications between 
miners working at the face is by a radio 
system. 



Under normal operating conditions 
the section foreman communicates by fixed 
phone to the shift foreman to request 
supplies and maintenance services, and to 
file his periodic productivity reports. 
Under emergency conditions he requests 
medical aid for personnel and reports 
hazardous conditions in his area. His 
primary concern is the safety and produc- 
tivity of his crew. 

The high acoustic noise level cre- 
ated by the mining machinery greatly 
reduces the effective communications 
between the foreman and his crew. This 
noise also interferes with the foreman 
receiving calls. Often a motorman must 
deliver a call-in message to the foreman 
when he is transferring haulage cars in 
his section. A standard procedure in 
some belt haulage mines is to turn off 
the conveyor system, thereby causing all 
the section foremen to call in. The 
working section crew primarily depends on 
the fixed pager phone system for direct 



The operational and safety advan- 
tages of communication capabilities are 
several and diverse. The shuttle buggy 
operator will be able to alert the con- 
tinuous mining machine operator of an 
impending roof fall. The shuttle buggy 
operators will be better able to coordi- 
nate their activities as they go in to 
dump on the belt or into the cars. The 
maintenance mechanic will be able to com- 
municate with the surface while working 
at a face machine. When maintenance on a 
face machine is required, the maintenance 
mechanic can be called directly from the 
troubled machine. 

3.4.1 Radio Systems 

The use of two-way radios can result 
in better coordination of section ac- 
tivities, especially during the movement 
of mobile machines that must work in 
concert with each other at the face area. 
In many cases the operators cannot see 
one another, but with a system of 



46 



communications they can still effectively 
work together. Safety will also be 
improved by better communication with 
isolated workers; for example, fan-hole 
drill operators in iron ore mines. 

Improved management can be realized 
by means of effective section communica- 
tion. The foreman can exercise better 
supervisory control, resulting in more 
efficient utilization of available per- 
sonnel. Another benefit is the reduction 
of unnecessary travel, an extreme burden 
when mining low coal or on longwall sec- 
tions. Repairmen, mechanics, and util- 
itymen can be quickly reached and dis- 
patched to their place of need at the 
time of need. In spite of these advan- 
tages, two-way voice communication using 
portable radios is only now becoming 
practical for use in underground working 
sections. This has been due to the lim- 
ited range that could be attained using 
the small handheld units. 

Almost anyone who has ridden in an 
automobile is familiar with the radio 
fade that occurs when a car enters a tun- 
nel. One might expect, then, that radio 
wave propagation would be very poor in 
mines, and it is still not feasible to 
design practical "wireless" portable ra- 
dios capable of full mine coverage, ex- 
cept possibly for the smallest mines. 
However, both theory and experience show 
that the propagation characteristics of 
radio waves in mine tunnels improve as 
the frequency increases into the UHF 
band. This is attributable to a wave- 
guide effect that is prominent when the 
wavelength of the radio wave becomes 
small compared with the cross-sectional 
dimensions of the tunnel. In the UHF 
band from 400 to 1,500 MHz, tunnel propa- 
gation is adequate to provide sectionwide 
radio coverage. 

Probably the most important factor 
that determines the ability of UHF radio 
waves to propagate in underground mine 
tunnels is the cross-sectional dimension 
of the tunnels. In general, a high, wide 



opening favors better radio wave propaga- 
tion. Figure 3-9 shows a comparison in 
the ability of 450-MHz UHF radio waves to 
propagate in high coal (7 feet) as 
opposed to low coal (3.5 feet), assuming 
a 16-foot-wide entry. The comparison 
also assumes that 2-watt UHF walkie- 
talkies are the source of signal. As 
indicated in figure 3-9, communication is 
possible for ranges up to 1,500 feet, 
along a straight entry in high coal, but 
the range drops to 400 feet in low coal. 
Of course the same principles apply to 
tunnels in noncoal mines. 

Corners also present obstacles to 
the propagation of UHF radio waves. For 
a path that includes one corner, ranges 
are reduced but improve if one of the 
radios can be moved closer to the corner. 
However, propagation around a second 
corner is usually poor. To help offset 
this corner effect, it is good practice 
to transmit from intersections when pos- 
sible, thus reducing the number of 
corners that have to be negotiated. Some 
other obstacles to radio wave propagation 
at ultrahigh frequencies are equipment 
such as shuttle cars and machines that 
reduce the cross-sectional area of the 
tunnels or entries. Table 3-1 shows that 
when shuttle cars are present, the range 
is typically reduced by 200 feet in high 
coal and by 50 feet in low coal. 



Portable radio 



Ui^ 



.J 



::5 






FIGURE 3-9. - UHF radio wave propagation in 
high and low coal. 



47 



TABLE 3-1. - Typical range reduction due to tunnel obstructions at 450 kHz 



Shuttle car ft. . 

Bends 

Permanent stoppings .... 
Longwalls ft. . 



High coal (7 by 16 ft) 

200 

Moderate to severe.... 

...do 

1,200 



Low coal (3-1/2 by 16 feet) 



50. 

Moderate to severe. 

Do. 
250. 



The range of effective conmunication 
can be substantially increased by the 
use, and judicious placement, of a re- 
peater, and in some applications, a radi- 
ating cable. When this is done, good 
communication can be established even un- 
der some of the worst conditions encoun- 
tered on working sections. Referring to 
figure 3-10, suppose the two radios la- 
beled A and B are out of direct radio 
range of each other. The repeater can 
function to bring the radios within range 
in the following manner. When radio A 
transmits on frequency Fl , the signal is 
picked up by the repeater, which ampli- 
fies and converts it to a different fre- 
quency (F2) and retransmits it at a 
higher power level. Radio B receives the 
retransmitted signal. In this way, com- 
munications from radio A to radio B and B 
to A are established. 

When the tunnels between the porta- 
ble radios are heavily obstructed by 
machinery or metal roof -support struc- 
tures, radiating (leaky coax) cable may 
also ■ be installed in the tunnel to pick 
up and carry the signals to and from the 




FIGURE 3=10. 
repeater. 



Extending range with radio 



repeater and portable radios. Figure 3- 
11 shows a sample cable installation for 
use with the repeater. In this case, the 
signals from radio A are picked up by the 
radiating cable itself, carried to the 
repeater, retransmitted as F2 signals on- 
to the cable, carried along by the cable, 
and leaked into tunnels where they are 
received by radio B. The reverse occurs 
for transmission from B to A. 

Even though a radio repeater such as 
shown in figure 3-10 can extend the oper- 
ating range of radios A and B, this still 
provides only local coverage such as a 
working section. However, a radiating 
cable-repeater system such as shown in 
figure 3-11 can extend the operating 
range for many miles. The limiting fac- 
tor in this case is the ability of a 
radio to transmit to and receive from the 
radiating cable. 

The implementation of a UHF radio 
system for a mine working section can be 
approached from the standpoint of a basic 



%x 



RADIATING CABLE 



fl- 



s 



RADIATING 

CABLE 

REPEATER 



^ 



FIGURE 3.11. 
cable. 



Extending range with radiating 



48 



"building block" philosophy as shown in 
figure 3-12. The fundamental building 
block is the UHF walkie-talkie radio 
itself. Several of these are sometimes 
all that is required for an effective 
sectionwide communication system. Usu- 
ally, however, certain portable accesso- 
ries are helpful to some miners; namely, 
speaker-microphone headsets, carrying 
vests, and remote handheld microphones. 

In some situations, it may be neces- 
sary to extend the range of communica- 
tions beyond that achievable when trans- 
mitting directly between portable units. 
This can be accomplished by adding a 
radio repeater to the system. A re- 
peater, which can effectively double the 
area of coverage, is essentially a signal 
booster that receives weak signals from 
distant radios and retransmits them at 
full power. Further enhancement is pos- 
sible by connecting the repeater to its 
antenna by means of a long length of 
special "radiating" cable that can be run 
through areas of poor coverage, such as 
the area along a longwall chockway. 
Radiating cable, also known as leaky coax 
or leaky feeder, allows signals to leak 
out of or into itself at a controlled 
rate. It effectively behaves as a long 
antenna that can guide radio waves around 
corners and bends. 



For a more comprehensive system, an 
interconnect may be installed to inter- 
face the radio system with other communi- 
cation systems , such as a pager phone or 
carrier phone system. This would be use- 
ful for paging key personnel in the 
section who are out of audible range of 
the section pager phone. However, the 
interconnect should operate only on a 
selective basis to avoid interference to, 
or by, the section radios. Hardware for 
implementing this radio interconnect is 
commercially available. 

UHF section radio has been used suc- 
cessfully on room-and-pillar working sec- 
tions at several mines. One such mine 
had a single conventional room-and-pillar 
section as shown in figure 3-13. The 
seam height was medium low (42 to 48 
inches). The section radio system con- 
sisted of walkie-talkie radios carried by 
various miners and a radio repeater lo- 
cated at a communications center (known 
as the communication sled) , which was 
placed near the power sled. The foreman, 
mechanic, shot firer, and a utility 
cleanup man were equipped with 2-watt ra- 
dios operating on two channels, 454 and 
457 MHz. The purpose of the repeater was 
twofold: (1) To extend coverage beyond 
the direct portable-to-portable range, 



To mine 
phone system 



^ 







T 






1 






1 










R 



















^ ,o 



Interconnect 
Antennas 

Radiating cable 
Radio repeater 

Portable accessories 
Portable radios 



FIGURE 3-12. - UHF radio system "building 
blocks." 




Seam height; 
42 — 48 inches 



FIGURE 3-13. - Section layout of room-and- 
pillar section. 



49 



and (2) to provide an interconnect 
between the radio system and a system of 
carrier phones, which were mounted on 
mobile machines and interconnected by 
means of the trailing cable conductors to 
the machines. It was thus possible to 
communicate between roving miners 
equipped with radios and machine opera- 
tors equipped with powerline carrier 
phones. Paging into the section radio 
system from a surface point was also pos- 
sible via a surface-to-section carrier 
phone link and a special interface in the 
communication sled. A low-frequency 
through-the-earth radio link between the 
surface and communication sled was also 
provided, as shown in figure 3-14. At 
this mine the portable radios by them- 
selves were usable over an area encom- 
passing more than half of the working 
section. With the repeater, sectionwide 
radio coverage was possible. 

A similar system was used at another 
room-and-pillar mine utilizing continuous 
mining machines. This section radio com- 
munication system also included machine- 
mounted carrier phones and a surface- 
to-section interface at a communication 
sled. Conditions at this mine were much 
more favorable for radio communication. 



CARRIER SIGNAL 



THRU - THE - EARTH SIGNAL 



UHF RADIO REPEATER 



SECTION 

PORTABLE 

RADIOS 






FOREMAN (PAGE) 



DRILLER CUTTER / BOLTER LOADER 



UTILITY CLEANUP 



SHOT FIRER 



WIRED LINKS 



-RADIO LINKS 



FIGURE 3-14. - Radio link between surface 
and communication sled. 



mainly because the seam height was 6 to 
7 feet. Direct portable-to-portable com- 
munication was generally good over an 
area encompassing up to three-quarters of 
the working section, although some dead 
zones were encountered where several 
corners had to be traversed. When the 
face was at maximum advance from the 
power center, the repeater located in the 
communication sled near the power center 
was out of reach of some portable radios; 
however, this could be rectified by 
extending the repeater antenna toward the 
face area by means of a coaxial cable. 

3.4.2 Longwall Mining 

With investment in a longwall face 
being in the millions of dollars, and 
production delays amounting to hundreds 
of dollars a minute, positive control and 
communication must be obtained. A repre- 
sentative longwall face crew might com- 
prise a foreman, two shearer operators, 
three chock advance miners , one or two 
mechanics, a headgate operator, and one 
miner at the tailgate. Voice communica- 
tion is frequently required between each 
of these crew members and between the 
headgate and tailgate. Since miners at 
the face must work under rather crowded 
conditions, starting and stopping the 
conveyor and mining machines are particu- 
larly crucial operations. It is essen- 
tial that everyone on the face knows what 
is happening. During maintenance opera- 
tions, frequent interchange of informa- 
tion between miners working at various 
points along the face is required. Good 
communication will improve the capability 
of describing and locating problems and 
coordinating maintenance efforts to 
reduce downtime. 

Figure 3-15, a longwall installation 
in low coal, dramatically illustrates the 
limited working space in longwall mining. 
The area consists of a long lateral tun- 
nel in which equipment may be easily 
damaged. Moreover, it is fatiguing to 
travel any appreciable distance to get to 
a phone placed along the face. Acoustic 



50 




FIGURE 3-15. = Typical conditions encountered 
in longwall mining. 

noise is also very high. Therefore, a 
communication system designed specifi- 
cally for longwall mining applications 
should meet the following requirements: 

1. Minimum size. 

2. Rugged. 

3. Direct acoustic sound along the 
face. 

4. Rugged cable and connector de- 
sign to survive in the harsh environment. 

5. Sufficient power to permit oper- 
ation along the maximum length of the 
longwall. 

6. Certain control and signaling 
features that can be incorporated into 
the phone system. 

U.S. longwall faces commonly use 
standard U.S. pager phones as a means of 
implementing interface communication. 
However, these systems do not provide an 
adequate face communication system. 
Major problems are as follows: 

1. The phones and cables are easily 
damaged owing to the close quarters and 
severe environment. 

2. Miners on the face may have to 
travel 50 to 100 feet to use phones; 
sometimes phones can survive only at the 
headgate or tailgate, which is marginally 



acceptable on a short face, say 250 feet, 
but unsatisfactory on faces as long as 
400 feet; in contrast, phones are placed 
40 to 50 feet apart in West Germany. 

3. The conveyor creates a high- 
noise environment, and shearer noise 
often makes it impossible for shearer 
operators to hear a page. 

4. Communication is required later- 
ally along the face, and U.S. pager 
phones have not been designed with this 
in mind. 

Several European systems, however, 
are available that have been specifically 
designed for longwall applications. Fig- 
ure 3-16 shows one type of phone, which 
has already been installed in a few U.S. 
longwalls and is reportedly well accept- 
ed. Figure 3-17 shows the main control 
unit, which is installed at the headgate. 
Some of the features of European longwall 
pager phone systems are pull-wire signal- 
ing, machinery lockout buttons, prestart 
warning, fault detectors (in some cases) 
which stop the machinery, blast-proof de- 
sign, and a central power supply at the 
headgate with standby batteries in the 
individual phone units. 

For the potential U.S. user, there 
are, of course, problems associated with 
this equipment. The first is expense. A 
10-phone system incorporating all the 
desired features may cost around $25,000. 
A 10-phone system with voice-only capa- 
bilities might cost only about $6,000 to 
$7,000, but this is still at least twice 




FIGURE 3-16. - Longwall phone. 



51 




^ 



FIGURE 3-17, - Main control unit. 

as much as a U.S. pager phone system. 
Secondly, there are a limited number of 
suppliers. Thirdly, a mine may have to 
either carry its own inventory or expect 
long lead times in getting spare parts. 
Finally, in-house maintenance skills have 
to be developed. However, given the high 
cost of a longwall system ($1 to $2 mil- 
lion) , a proper understanding of the 
value of a good phone system in reducing 
downtime indicates that these systems are 
still worth considering. 

With any system, certain individuals 
should be able to communicate from any 
location along the chockway without the 
fatiguing ordeal of crawling to a phone. 
This requirement cannot be totally met by 
any wired phone system, and with some 
exceptions wireless radio for longwalls 






- 300 TO 600 FT TYPICAL 



LONGWALL FACE 



^... 



d 



FIGURE 3-18. - Repeater-based UHF radio sys- 
tem layout for longwalls. 



is not feasible at ultrahigh frequencies. 
Table 3-2 summarizes the important points 
regarding the design and implementation 
of a longwall UHF radio system. However, 
cable-aided UHF radio is feasible and may 
be another choice for obtaining the 
linear tunnel coverage required on a 
longwall section. 

On shorter faces , a radiating cable 
extending along the length of the long- 
wall and passively terminated at each end 
with a suitable antenna can provide face 
coverage without a repeater. A radio 
repeater, connected to the cable at one 
end, may be needed on longer faces, or 
when coverage to the head entry outby the 
headgate is required. A repeater-based 
configuration for a longwall UHF radio 
system is shown in figure 3-18. In this 
system, good radio coverage can be ex- 
pected along the face area and into the 
head entry for several hundred feet. If 
the repeater should fail, direct communi- 
cation between portable radios is still 
possible at reduced range. This system 
can be implemented using commercially 
available battery-operated hardware that 
is also MSHA approved (fig. 3-19). 



TABLE 3-2. - Ranges of completely wireless communication system 
for longwalls at 450 MHz 



Type of roof 


Range with no 


Range with shearer machine, 


ft 


support 


machine, ft 


High coal 


Low coal 






High coal 


Low coal 




Chocks 

Shield 


300 
1,000 


150 
150 


100 
300 


50 
50 



52 



The reason for this is because the solu- 
tions to the communication requirements 
in these systems are similar. Nontrolley 
haulage systems include the following: 

Rail vehicles with self-contained 
power sources (battery or diesel 

powered) . 

All rubber-tired vehicles. 



FIGURE 3=19. . MSHA=approved UHF repeater 
and battery unit, with 'hardhat" antenna and 
portable radios. 

3 . 5 Hau 1 ageway s 

Operators of vehicles in underground 
haulageways must be able to communicate 
with one another and with other areas in 
the mine to improve safety and increase 
production. 

The type of systems that can be 
implemented to meet the communication 
requirements of underground haulageways 
depends upon the method of haulage it- 
self. From the standpoint of communica- 
tions, haulage systems can be considered 
as either trolley or nontrolley. 

Trolley haulage, as used in this 
manual, means vehicles that are tracked 
(ride on rails) and are electrically 
powered from an overhead trolley wire. 
In mines using this type of haulage, a 
carrier phone system using the trolley 
wire is almost always used to satisfy the 
communication requirements. The trolley 
wire and tracks serve as the carrier 
current path. Methods, techniques, and 
ways of improving carrier phone systems 
are given in section 3.5.1. Special con- 
trol, monitoring, and communications 
requirements involved when moving off- 
track equipment under an energized 
trolley wire are described in paragraph 
4.4.3d of chapter 4. 

All other forms of haulage systems 
are grouped into the nontrolley category. 



Communication systems applicable to 
nontrolley haulage systems are described 
in section 3.5.2. 

3.5.1 Trolley Haulage 

As previously mentioned, carrier 
phones are usually used to satisfy the 
communication requirements in haulage- 
ways where rail vehicles that draw power 
from an overhead trolley wire are used. 
One reason carrier phones have become so 
popular is that they operate over the 
existing trolley wire dc power circuits 
to provide two-way voice communication 
between the tracked vehicles and with 
fixed stations in the mine. No addi- 
tional wires or cables must be installed 
in the mine. In underground mining, 
these carrier systems are used exten- 
sively for traffic control of the tracked 
haulage equipment and personnel carriers. 
These phones are FM push-to-talk 
transmitter-receiver units designed for 
common talk (party line) operation. 

Carrier frequency couplers consist- 
ing of bypass capacitors are used to pro- 
vide continuity of the RF signal path 
between sections of trolley served by 
different dc power centers. These car- 
rier systems typically operate in the 
60- to 200-kHz range. (See section 2.4 
for basic theory of operation of carrier 
current phone systems.) 

The carrier phone located on each 
tracked vehicle is primarily used for 
control of vehicle traffic. All vehi- 
cles are kept in communication with each 
other and the dispatcher over the single- 
channel (party line) carrier phone 
system. This single-channel network 
keeps the dispatcher and all motormen in 



53 



continuous contact with one another so 
that right-of-way and the disposition of 
haulage cars will be known to all. One 
inherent advantage of the trolley carrier 
phone system is that it is a party line 
system. In certain applications, this 
would be a disadvantage since private 
communication channels are not available. 
For haulageway traffic control, however, 
it is beneficial if each motorman does 
hear conversations between other motormen 
and the dispatcher. This phone system 
also allows the dispatcher to notify all 
motormen of any mine emergency. The two 
drawbacks to this system follow: 

Trolley wire power failures, which 
cause the carrier communication system to 
go dead unless backup batteries are 
installed. 

Dead zones, which are sections of 
track where the phone is inoperative due 
to excess electrical noise, excess atten- 
uation of signal strength, or standing 
wave effects. 

The first drawback, loss of communi- 
cation due to power outage, can be cor- 
rected by the use of backup batteries in 
each vehicle (required by law if the 
carrier current system is the only com- 
munication system in the mine) . The 
backup batteries would normally be 
trickle-charged to full capacity and then 
maintained at full charge. In the event 
of a power failure on the trolley wire, 
the backup batteries would automatically 
power the carrier phones and allow for 
voice conmiunications for many hours. 
Because the haulage system is vital to 
mine operations, extended power outages 
on the trolley line are not tolerated. 
Any trolley power failure is immediately 
recognized and corrected as soon as pos- 
sible. Thus, communication outages due 
to power failures are minimal. 

The second problem associated with 
some carrier phone systems is that of 
"dead zones." There are areas where two- 
way communication between a vehicle and a 
dispatcher or between vehicles is not 
possible. Dead zones are caused by 
extreme attenuation of signals, excess 
noise, standing waves, and/or inadequate 



squelch control. The most significant 
of these causes is the extreme attenua- 
tion of the carrier phone signals on the 
trolley wire-rail. The trolley wire- 
rail is a poor radio frequency trans- 
mission line for several reasons, the 
most dominant of which is the presence 
of many bridging loads between the trol- 
ley wire and rail. Branches and the lack 
of good electrical terminations contrib- 
ute to the problem as well. The bridg- 
ing loads, which both absorb and reflect 
power, comprise such items as personnel 
heaters, rectifiers, pumps, haulage ve- 
hicles (motors) , locomotive and jeep 
lights, insulators, signal and illumina- 
tion lights, and even the carrier phones 
themselves. 

Because of the importance of good 
communications in the haulageway s and 
because a large number of mines use car- 
rier phones to meet these requirements, 
many programs have been sponsored to 
improve trolley carrier phone systems. 

One program was designed to (1) 
identify poor performance of trolley car- 
rier phone systems, (2) assess the causes 
of poor performance and classify them on 
the basis of equipment, coupling, or 
transmission problems, and (3) propose 
and verify the means to overcome these 
problems. 

Figure 3-20 illustrates the signal- 
attenuation rate for an "unloaded" 
trolley wire-rail transmission line; a 
band of rates is shown because the actual 
rate depends on the conductivity of the 
surrounding medium. If an attenuation 
rate of 1 dB/km is used, a trolley car- 
rier phone line having an allowable 
transmission loss of 70 dB (from 25 volts 
to 8 millivolts) yields a communication 
range of 70 km (43 miles). This perform- 
ance, in the absence of bridging loads, 
can be compared with that of a sample 
trolley wire-rail loaded as illustrated 
in figure 3-21. Here, just three bridg- 
ing loads of modest value (typical of 
vehicles and personnel heaters) reduce 
the signal 55 dB over a distance of just 
4,500 feet. The figure also shows the 
signal level that would exist over the 
same distance on a properly terminated 



54 



2 — 







^ 



100 200 

FREQUENCY (kHz) 



400 



800 



FIGURE 3-20, - Signal-attenuation rate for an 
'unloaded' trolley wire-rail transmission line. 



trolley wire-rail without bridging loads. 
With such signal reductions, it is easy 
to see why it is difficult to obtain 
long-range transmission of carrier sig- 
nals using the trolley wire. 

There is one approach that appears 
to have merit in overcoming the excep- 
tionally high attenuation rates that can 
be expected on the trolley wire-rail — the 
dedicated wire technique. 

3.5.1a Dedicated Wire 

The best approach to overcoming the 
extremely high attenuation rates imposed 
by bridging loads is a single-purpose, or 
"dedicated," wire. The characteristics 
of an unloaded trolley wire-rail are such 
that it forms a low-loss transmission 
line. Therefore, a separate wire strung 
in an entryway, with the same rail return 



-40 
-50 
-60 
-70 




• DATA 
— THEORY 



DISTANCE ALONG LINE - FEET 



SIMULATED LENGTH = 4460 FEET 



^ 



-19 
2000 pF 
CAPACITORS 



SHUNT LOADS 




• DATA 
THEORY 



DISTANCE ALONG LINE - FEET 

FIGURE 3-21, - Signal level on simulated trolley 
wire-rail, 

path as the trolley wire but unloaded by 
any bridging loads, would similarly serve 
as a low-loss line. Such a configuration 
forms a three-wire transmission line. 

Studies have shown that the primary 
mode of propagation for such a configura- 
tion is a low-loss mode supported by the 
dedicated wire, with the rails serving as 
the return signal path. The signal im- 
provements that can be expected from such 
a configuration are illustrated in figure 
3-22, which shows the voltage signal 
strength versus the distance along a 
heavily loaded trolley wire-rail with a 
parallel dedicated wire. 

Four separate conditions of trans- 
mission and reception are shown in 



55 




high-voltage trolley wire and is 
influenced by its loads. 



not 



TRANSMIT ON TROLLEY OR 
DEDICATED WIRE AND 
RECEIVE ON THE OTHER 



TRANSMtT AND RECEIVE ON 
TROLLEY WIRE (WITH A 

DEDICATED WIRE)- 



FIGURE 3-22. - Voltage signal strength versus 
distance along heavily loaded trolley wire^rail 
v/ith a parallel dedicated wire. 

figure 3-22. For example, if the dis- 
patcher transmits on the dedicated wire 
and the motor operators receive on the 
trolley wire, then the dispatcher's 
transmission will produce a curve of 
trolley signal level versus distance like 
curve B. At a distance of approximately 
12.5 miles the dispatcher's signal shows 
only a loss of 50 dB. The remaining sig- 
nal level is entirely adequate for opera- 
tion of the carrier phones, since the 
allowable transmission loss is about 70 
dB. 

The crosshatched area between 
curves C and D illustrates the improve- 
ment that can be obtained when both the 
dispatcher and motor operators still 
transmit and receive on the trolley wire 
but when a dedicated wire has been in- 
stalled along the haulageway. 

Curve A shows the signal loss be- 
tween fixed base stations, both of which 
can transmit and receive over the dedi- 
cated wire. 

The dedicated wire is installed with 
no direct connection to the trolley wire- 
rail; however, a strong electromagnetic 
coupling exists between the dedicated 
wire-rail and the trolley wire-rail, 
simply because of their physical proxim- 
ity. Thus, the dedicated wire is not 
jeopardized by direct coupling to the 



Tests indicate that excellent re- 
sults can be obtained with a dedicated 
wire. The dedicated-wire concept permits 
installation of a transmission line with 
controlled branching that can be termi- 
nated to avoid standing-wave patterns. 
The price paid is that the wire has to be 
installed and maintained in a haulageway. 

A recent study (26)^ recommends that 
the dedicated-wire method is usually the 
most effective and practical way of up- 
grading trolley carrier phone systems. 

. Another research program provided a 
set of five guidelines for operat- 
ing personnel to improve their carrier 
phone systems. These guidelines give 
detailed instructions for installing 
trolley carrier phone equipment onboard 
mine vehicles and at the dispatcher's 
room, converting a rail haulage trolley 
wire-rail and feeder system into a 
functional carrier-frequency-transmission 
line, checking the performance of the 
trolley carrier phone system, and using 
portable test equipment to aid in system 
maintenance. A detailed description of 
the conclusions and the recommendations 
set forth in these guidelines are pre- 
sented in chapter 6 of this manual. 

Rather than offering detailed com- 
ments on the contents of each of these 
guidelines here, we focus on just one 
aspect of the guideline concerned with 
converting the trolley wire-rail into an 
efficient transmission line. In the pre- 
ceding discussion on the causes of poor 
performance, the extremely poor propaga- 
tion characteristic of the trolley wire- 
rail was cited. Apparently, this poor 
propagation dominates in determining the 
performance of trolley carrier phone sys- 
tems. Thus, it is appropriate that seri- 
ous consideration be given to determining 
the signal and noise levels on each 
trolley wire system. Signal strength and 

^ Underlined numbers in parentheses 
refer to items in the bibliography at the 
end of t±iis chapter. 



56 



electromagnetic noise level measurements 
should be made at points along the 
trolley and noted on a mine map. The 
procedure is simple. The dispatcher's 
transmitter is used as the signal source, 
and both the strength of the signal along 
the haulageway and the corresponding 
noise level are measured. This measure- 
ment is conveniently made by equipping a 
jeep with a tuned voltmeter. The jeep 
moves along the haulageway, stopping at 
intervals of about 2,000 feet. The opera- 
tor calls the dispatcher and asks for a 
10-second keying-on of his transmitter. 
The received voltage on the tuned volt- 
meter is noted on a mine map, and the 
noise level is also noted. This map then 
identifies regions of the mine where ex- 
cess noise may be the problem, as well as 
regions where weak signal levels cause 
problems. The map also aids in identify- 
ing the key bridging loads branches, or 
unterminated lines that can cause prob- 
lems. This signal- and noise-mapping 
process is the key to identifying the ma- 
jor causes of poor signal reception in a 
particular mine. 

Once the probable source of diffi- 
culty has been identified, the remaining 
part of the guidelines can be consulted 
to determine possible ways of treating 
the problem. For example, if a rectifier 
is affecting signal propagation, the 
guidelines provide three different ways 
to treat the rectifier to reduce the 
problem. 



means of dispatcher-vehicular communica- 
tions. Most problems associated with 
these systems are transmission line re- 
lated; a trolley line was never intended 
to be a good communications line, and it 
certainly is not. However, techniques do 
exist for improving overall communica- 
tions. These techniques can be easily 
implemented, and the results are often 
excellent. 

3.5.2 Nontrolley Haulage 

An increasing number of both newly 
developed and older mines have been aban- 
doning tracked trolley vehicles and are 
conducting their haulage, maintenance, 
and personnel transport operations with 
other types of vehicles. Obviously, com- 
munications to and from vehicles operat- 
ing independently of a trolley wire can- 
not be implemented by the trolley carrier 
phones discussed in the previous section. 

Communication systems required for 
battery- or diesel-powered rail vehicles 
or rubber-tired vehicles have one common 
characteristic. Because these vehicles 
are not physically attached or connected 
to any wiring or other conductor in the 
mine, some form of radio link must be 
utilized to establish the final voice 
link with the vehicle. If voice communi- 
cation exists from a nontrolley vehicle, 
then an antenna-radio link of some form 
must be used to replace the direct con- 
nection provided by the trolley wire. 



3.5.1b Summary 

Communications with moving tracked 
vehicles in a rail haulage mine pose a 
difficult problem. These communications 
take place from dispatcher to vehicles or 
from vehicle to vehicle via the trolley 
line, which is a very poor communications 
line. As a result, dead spots and high- 
noise areas can occur anywhere along the 
line; also, signal strength can decrease 
simply as a function of distance. 

Although trolley carrier phone sys- 
tems leave much to be desired for haul- 
ageway communications, the fact remains 
that they do represent one practical 



Several methods exist for providing 
communication between nontrolley mining 
vehicles. Studies have been conducted of 
high-frequency systems utilizing the so- 
called leaky-coax cable to carry signals 
throughout a mine. Other studies in the 
wireless radio area have shown that at 
medium frequencies, signals follow the 
existing mine wiring for great distances. 

3.5.2a Leaky-Coax Systems 

A leaky coax is a special type of 
coaxial cable that allows radio frequency 
signals to leak into and out of itself. 
With this type of cable, signals can be 
transmitted to and received from mobile 



57 



radio units near the cable. Leaky coax 
is therefore ideally suited for haulage 
applications. In effect, the cable 
guides the radio signals down the tunnel 
(fig. 3-23). Although signal strength 
does attenuate along a cable run, re- 
peaters or in-line amplifiers can be used 
to extend the range of coverage. Several 
techniques have been used: 

1. Borrowing from conventional 
mobile radio communications practice, 
individual fixed-base stations can be 
installed at intervals as necessary to 
provide the total range, all stations 
being under a common remote control with 
the first. Such a system has been in use 
at a British mine since 1970. 



an audio return line is required and, 
when branches are required, the system 
can become complex. 

3. Multiple-frequency repeater 
schemes (fig. 3-25) have also been used 
successfully; the simplest uses one 
transmitter and one receiver. 

Communication benefits of a leaky- 
coax system are typified by one system 
developed for an iron ore mine (block- 
caving operation) using rubber-tired, 
diesel-powered vehicles. The system 
chosen to satisfy the communication 
requirements at this mine consisted of a 
UHF leaky-coax system. Figure 3-26 is a 
simplified diagram of the system. 



2. A series of one-way in-line 
repeaters, such as the daisy-chain system 
shown in figure 3-24, is effective; it 
does have a slight disadvantage in that 



AREA OF RADIO COVERAGE 
FREQUENCY - 420 MHz 
SIGNAL STRENGTH (VOLTAGE OR ELECTRIC FIELD! 




. " 2000 FEET 



LIMIT OF COMMUNICATIONS IN HAULAGEWAV 



FIGURE 3-23. - Cable "guiding" radio signal 
down a tunnel. 



t> — — > 



LEAKY FEEDER WITH ONE-WAY REPEATERS 



i 



FIGURE 3-24. - Blockdiagramof a daisy-chain 
repeater system. 



In addition to providing communica- 
tion to personnel carriers, maintenance 
and production vehicles, and the 
ambulance (fig. 3-27) , the system pro- 
vides communications for roving miners, 
foremen, fan-hole-drill operators, and 
supervisors. Communication requirements 
were satisfied by using (1) UHF wire- 
less radio, (2) a radiating coaxial cable 
or "leaky" transmission line to carry 
the signal throughout the haulage and 
subdrifts of the mine, (3) interconnected 
VHF and UHF repeaters, (4) portable 
transceivers, and (5) vehicle-mounted 
transceivers. 



I- 



REQUENCY, 420 ^ 



REPEATER OR BASE 
STATION WITH 
SURFACE INTER- 
CONNECT 



ANTENNA TERMINATION 



SECTION eRANCH OFF 



^. 



HAULAGEWA' 
1.200 METERS 



ROVING HANDE TALKIE 



FIGURE 3-25. - Two-frequency repeater concept. 



58 



FAN HOLE 

DRILL 

OPERATOR 




AMBULANCE 



MAINTENANCE 
VEHICLE 



FIGURE 3-26. 
system. 



RF leaky-coax communication 




FIGURE 3-27. - UHF mobile radio mounted on 
underground ambulance. 

The mine was divided into two RF re- 
gions, with each region (zone) containing 
one UHF-VHF repeater station and associ- 
ated runs of leaky coax. The system of 
cables effectively wires the mine for UHF 
signals between portable and mobile un- 
its. Each repeater station can receive 
and transmit signals on the cable at both 
UHF and VHF. VHF signals are used on 
the cable as a communication link between 
the stations, while the UHF is used for 
the communication link to and from the 
portable and mobile units. The two UHF 



repeaters transmit on F2 and receive on 
Fl as shown in figure 3-28. The VHF re- 
peaters use frequencies F3 and F4 to in- 
terconnect the two UHF zones. Each UHF- 
VHF repeater station can simultaneously 
transmit and receive on both UHF and VHF. 

The mobile radios transmit on Fl and 
receive on F2. All information therefore 
goes to the repeaters, then back to all 
other units. The portable radios are 
also capable of transmitting on F2 and 
therefore are able to talk to one another 
without the repeaters on a local simplex 
basis. Audio control lines are provided 
from the crusher console to repeater 
station A and from the surface guardhouse 
to the shaft bottom station, thus provid- 
ing system access from two hardwired 
locations as well as an important emer- 
gency link to the surface. 

As an example, suppose that a mobile 
radio in zone A wants to talk to a roving 
miner equipped with a portable radio in 
zone B. The operator in zone A keys 
his radio and talks into his microphone 
to transmit his message on UHF Fl . The 
UHF signal is coupled to the leaky coax 
and travels to the UHF-VHF repeater in 
zone A. Repeater A rebroadcasts the mes- 
sage back to zone A on UHF F2 and also 
sends the message to the repeater in 
zone B on VHF F4. This signal travels on 
the coaxial cable to UHF-VHF repeater B 
where it is picked up by the VHF re- 
ceiver. The signal is then converted to 
UHF F2 and routed onto the leaky coax for 
distribution in zone B. 



V 

a — ^ 



'^ 



ri 

d 






REPEATER A 



REPEATER B 



FIGURE 3-28. . UHF-VHF repeoler system. 



59 



For this type of Installation, 
specifications recommended that the cable 
be supported every 5 feet. To avoid 
installing a large number of anchors in 
the rocks, a 3/16-inch steel messenger 
wire was attached at 20-foot intervals to 
roof-bolt-supported T-bars (fig. 3-29) . 
The cable was then strapped to the mes- 
senger wire with standard cable ties. 

A vehicle-mounted work platform, 
which could be mechanically raised or 
lowered and which was equipped with a 
frame for supporting cable reels, facili- 
tated cable installation. The factory 
cut the cable to predetermined lengths, 
installed connectors, and tagged the 
cable with location identifiers. In mine 
areas that were so far removed from the 
main cable that radio transmissions could 
not be established, a stub cable was 
installed with one end connected through 
a power divider to the main cable and the 
other end terminated with an antenna. 

3.5.2b UHF Reflective Techniques in 
Underground Mines. 

UHF radio (300 mHz to 3 gHz) is the 
only way of achieving true radio propaga- 
tion in an underground mine. Propagation 
is possible because the mine entries 
function as waveguides that confine the 
transmitted energy. Several thousand 
feet of range, line-of -sight, is often 
possible without leaky feeder cables if 
the entries are large enough. 




FIGURE 3=29o 

messenger wire. 



Radiax cable installation on 



However, the nature of UHF is such 
that propagation around bends and corners 
introduces tremendous signal losses. In 
this regard, it is similar to the trans- 
mission of light and, like light, it can 
be reflected by flat metallic surfaces. 
These characteristics of UHF make possi- 
ble a whole-mine communication system 
that does not rely on leaky feeder ca- 
bles. The Bureau of Mines evaluated such 
a system in an underground limestone mine 
that had large dimension haulageways. A 
UHF reflective radio system was designed 
to allow communication between super- 
visory personnel, maintenance personnel, 
haulage operators, and surface opera- 
tions. Communication was also provided 
between the hoist operator and slope car 
occupants. A closed circuit television 
(CCTV) system allowed continuous, remote 
visual monitoring of critical belt trans- 
fer points and underground dust disposal 
operations. 

The Black River Mine was selected as 
a typical metal-nonmetal room and pillar 
mine. It is nearly 4,000 feet in diam- 
eter, 650 feet deep, and has essentially 
straight crosscuts approximately 30 feet 
wide and 24 to 40 feet high with pillars 
approximately 35 feet square. Entry is 
through a 2,200-foot slope by means of a 
single drum, hoist-powered flat car and 
enclosed man carrier. Rubber tired, die- 
sel powered mine vehicles travel along 
designated haulage and travel roads from 
the active faces on the mine's perimeter 
to two rock crushers, the shop area, and 
the base of the slope. 

Tests of communication between hand- 
held, 2-watt UHF transceivers in the room 
and pillar limestone mine were satisfac- 
tory for approximately 2,000 feet through 
straight haulageways but the range of 
comminication at right angles to haulage- 
ways into Intersecting crosscuts was 
quite limited. It was evident that the 
radiation from the transceivers was not 
being reflected by the limestone pillars 
into the intersecting crosscuts. 

To improve communication in inter- 
secting haulage roads, 27 passive re- 
flectors were designed and installed at 



60 



major intersections. The reflectors were 
formed from 4- by 8-foot sheets of No. 16 
gage soft aluminum sheet that were sus- 
pended from wires attached to roof plates 
and bolt anchors. The roof height was 
sufficient to allow haulage vehicle 
clearance at each installation. Two dis- 
tributed antenna systems were designed 
to provide either an antenna or reflec- 
tor at the intersection of principal 
haulage and travel roads. Each antenna 
system consisted of approximately 1,200 
feet of 7/8-inch low loss foam dielectric 
transmission line which fed, through 2:1 
power dividers, four 5-dB gain mobile 
whip antennas that were suspended at 
intersections. 

A leaky coaxial cable anatenna sys- 
tem along the principal haulage and 
travel roads was considered but rejected 
because the range of communication at 
right angles to the leaky cable into in- 
tersecting crosscuts would have been much 
less than the range of communication from 
antennas. The leaky cable system is 
appropriate for long tunnels but not for 
intersecting roads in a room and pillar 
geometry. Also, a leaky cable would be 
more expensive. 

One central, or "backbone" coaxial 
cable carried 60 Hz power, radio signals, 
and CCTV signals for the entire system. 
Redundant routing of the backbone cable 
insured continued system operation in the 
event of a cable break. 

Fourteen 11-watt mobile radios 
equipped with automatic identification 
and emergency alarm encoders were in- 
stalled on vehicles in the mine. The en- 
coders are used on mine haulage trucks to 
automatically send three status signals; 
truck bed up (dumping) , truck bed down, 
and hot engine. This information is dis- 
played by number codes along with the 
truck's identification number on display 
units in the engineering office above 
ground and the mine foreman's office un- 
derground. A record of all calls, sta- 
tus, and alarm messages is automatically 
printed in the engineering office. 



Fifteen 2-watt portable transceivers 
equipped with automatic identification 
and emergency alarm encoders are used by 
mine department heads, foremen, and per- 
sonnel in the mine. 

Signal margin measurements of the 
base-repeater station signals along 
haulage and travel roads were made after 
both distributed antenna systems had 
been completed, which demonstrated that 
approximately 75% of the mine area re- 
ceived satisfactory signals, but active 
mining areas along the perimeter of the 
mine were not adequately served. The 
distributed antenna system would have 
to be extended to serve additional anten- 
nas near the mine faces. However, the 
cable attenuation would drastically re- 
duce the power radiated from the antennas 
and the signals received from the mobiles 
and portables so that very little im- 
provement would be realized. Additional 
base-repeater stations were considered; 
however, the added complexity and cost of 
multiplexing equipment and for extending 
the backbone cable control system stim- 
ulated the development of a low-cost, 
two-way multichannel signal booster sys- 
tem. A prototype signal booster was con- 
structed and tested. Six amplifier sig- 
nal boosters, 10,000 feet of cable, and 
16 additional antennas were installed in 
the mine. Subsequent signal measurements 
showed adequate coverage of all desired 
areas. 

3.5.2c Dedicated-Wire Radio Systems 

It is possible to use trolley car- 
rier current techniques and hardware to 
communicate with vehicles that do not use 
a trolley line, such as battery-powered 
railed or rubber-tired vehicles. How- 
ever, in this case, a "dedicated wire" is 
essential for proper operation. Such a 
system is shown in figure 3-30. 

The dedicated wire takes the place 
of the trolley line. However, since the 
carrier phone on the jeep communicates 
with the dedicated wire by a loop 
antenna, instead of touching it like it 



61 



DISPATCHER 



y 



LOOP ANTENNA 



( ) 
















CARRIER 
PHONE 











"^ 



^S7 



^S^ 



BATTERY POWERED JEEP 

FIGURE 3-30. - Dedicated-wire radio system. 

would a trolley line, this system is rel- 
atively inefficient. In general, it is 
usually necessary for the loop antenna to 
be rather close to the dedicated wire for 
communications. This problem is caused 
in part by the fact that carrier phones 
were never intended to use antennas, and 
cannot operate at high enough frequencies 
to make this approach efficient. 

However, research has shown that 
such a system operates well if medium 
frequencies are used. These frequencies, 
usually around 500 to 900 kHz (as opposed 
to 100 kHz typical of trolley carrier 
phones) can operate with loop antennas 
very efficiently. Considerable research 
is being done by industry and the Bureau 
of Mines to develop whole-mine medium- 
frequency systems. 

3. 5. 2d Wireless Radio System 



discussing the advantages and disadvan- 
tages of each technique, the subject of 
radio interference and signal attenuation 
in underground mines must be considered. 

3.5.2d.l Interference 

During normal operation, the machin- 
ery used underground creates a wide range 
of many types of intense electromagnetic 
interference (EMI), which is a major lim- 
iting factor in the range of a radio com- 
munication system. EMI generated in 
mines is generally a random process. 
Therefore, the most meaningful parameters 
for EMI are statistical ones. In work by 
the National Bureau of Standards, time 
and amplitude statistics have been used 
in order to unravel the complexities of 
EMI noise in mines. Without going into 
the details of data collection techniques 
or advanced statistical analysis, we will 
summarize the findings and conclusions on 
EMI affecting haulageway radio communica- 
tions. Figure 3-31 shows interference 
levels measured along haulageways in four 
different mines. 

The EMI noise levels shown for 
mine 1 are based on measurements made in 
a mine located in southwestern Pennsyl- 
vania. Room-and-pillar techniques were 
used with mining accomplished using a 
continuous miner, shuttle cars, and elec- 
tric trolley rail haulage. 



An obvious advantage of any true ra- 
dio system is that the system requires no 
transmission lines or cables. These sys- 
tems are immune to communication outages 
caused by line breaks due to roof falls 
or damage from machinery. However, the 
underground mining industry cannot take 
for granted the utilization of wireless 
communications as can their counterparts 
on the surface. As an example, at CB ra- 
dio frequencies, reliable communication 
in a mine entry is limited to about 100 
feet. Two options are available to the 
underground mine operator: (1) To use 
frequencies that are high enough to uti- 
lize the entries as waveguides, or (2) to 
use frequencies that are low enough that 
propagation through the earth, or by par- 
asitic coupling, can be insured. Before 



^ HIGH 

INTERFERANCE 
AREA 


LOW 

INTER 

AREA 


ERANCE 






i 










\ 


L 




MINEl 

HIGH COAL, RAIL HAULAGE 








T 


S^ 


/ 




-BLOCK CAVING 




MINE 2 

LOW COAL. BELT HAULAGE 


MINE 3 

IRON MINE 




MINE 4 
HIGH COAL 

1 


BELTH 


<> 


/ 




_ 


AULAGE 

1 1 1 


1 


1 1 


- 



100 120 



FREQUENCY^Hz 



FIGURE 3-31. - Interference levels measured 
along haulageways in four mines. 



62 



The majority of noise measurements 
were made in an area where the overburden 
ranged from 600 to 900 feet. The entire 
mine, including all machinery, is powered 
by 600 volts dc. All conversion from 
alternating to direct current is done on 
the surface, with the result that no ac 
power is brought into this mine. 

The EMI noise levels shown for 
mine 2 are based on measurements made in 
West Virginia. The coal in this mine 
occurs in a narrow seam, approximately 
3 feet thick, and is called low coal. 
The measurements were made in the two 
sections of the mine using the longwall 
mining technique where overburden was 
between approximately 900 and 1,500 feet. 
The mine also had seven conventional 
room-and-pillar sections. This mine used 
250-volt dc trolley haulage to carry coal 
out of the mine, and ac-powered conveyor 
belt haulage from the section to the 
trolley. All of the section longwall 
mining equipment was ac powered, with the 
exception of a dc-powered cable winch 
which was used occasionally to advance 
portions of the longwall equipment. The 
face and associated longwall equipment 
were 450 feet long. There were a total 
of six electric motors in the section 
ranging from 15 to 300 hp. The shear and 
face conveyor were powered by 950 volts, 
and the stage loader and hydraulic pumps 
operated from 550 volts. The stepdown 
transformer supplying these voltages was 
kept approximately 150 to 700 feet back 
from the face and was supplied with 
13,200 volts. 

The EMI noise levels shown for 
mine 3 are based on measurements made in 
a Pennsylvania iron mine. The level 
where measurements were taken was approx- 
imately 2,300 feet below the surface. 
The ore body is a large, flat, oval de- 
posit about 300 feet thick, mined by un- 
dercutting and allowing the ore to cave 
into drawpoints called entries. Air- 
cooled, V-8 diesel-powered, rubber-tired, 
load-haul dump (LHD) vehicles were used 
to haul the ore to the underground crush- 
er and dump it into the ore crusher; it 



was transported by conveyor belt horizon- 
tally 825 meters, then lifted to the 
surface by a skip. The other types of 
haulage equipment used in this mine also 
were diesel powered and rubber tired. 
All haulageways were through reliable 
rock or were heavily reinforced with con- 
crete and steel. The mine used a mixture 
of incandescent, mercury-arc, and fluo- 
rescent lighting. 

The noise levels for mine 4 were 
made in a West Virginia mine where room- 
and-pillar mining techniques were used. 
The measurements were made primarily in a 
section where overburden was approxi- 
mately 600 to 900 feet. Mining was 
accomplished using a continuous miner, 
head-loader, shuttle cars (buggies), con- 
veyor belt, and electric trolley haulage. 
The electric trolley and the shuttle cars 
were powered by 300 volts dc. All other 
equipment, including fans and rock dust- 
ing machines, was ac powered. 

The noise measurements taken in 
haulageways of these mines tended to show 
magnetic field strengths typically 60 to 
70 dB pA/m up to a few kilohertz, which 
then decreases sharply above 8 to 12 
kHz. 

As seen in figure 3-32, the EM noise 
amplitude decreases with increasing fre- 
quency; however, three propagation mech- 
anisms must be considered: (1) Through 
the earth, (2) through the entries sup- 
ported by metallic structures and con- 
ductors, and (3) through the entries 
where they serve as a "waveguide." For 
propagation through the entries, it would 
appear, from the data presented, that se- 
lection of frequencies much greater than 
100 kHz would be desirable. 

For situations in which the propaga- 
tion is directly through the earth, at- 
tenuation (signal loss) increases as fre- 
quency is increased. Because of lower 
attenuation at lower frequencies, better 
signal-to-noise ratios exist at low fre- 
quency despite the higher noise levels. 



63 



GENERAL 

LOSS 

IN SIGNAL 

STRENGTH 




FIGURE 3-32. = EM noise amplitude decrease 
with increasing frequency. 

3.5.2d.ii Signal Attenuation in the 
Haulageway 

As a radio signal travels dovm 
a haulageway or tunnel, its strength 
decreases. Typical signal attenuation 
along a straight tunnel, for three dif- 
ferent radio frequencies, is shown in 
figure 3-32. Transmission loss may be 
combined directly with transmitter power 
and antenna gains to determine the re- 
ceived signal for any candidate UHF sys- 
tem. In terms of transmission loss, a 
pair of 1-watt UHF walkie-talkies has a 
range of 143 to 146 dB. 

Significant propagation characteris- 
tics apparent from figure 3-32 are — 

Attenuation (in decibels) Increases 
nearly linearly with increasing distance. 

Transmission loss decreases signif- 
icantly at a given distance as the fre- 
quency is increased. 

3. 5. 2d. ill Signal Attenuation Around 
Corners 

Observed signal attenuation around 
a corner is also shown in figure 3-32. 
Corner attenuation is plotted in deci- 
bels relative to the signal level ob- 
served in the center of the main tunnel. 



Figure 3-32 shows that signal attenuation 
around a corner is considerable. Because 
of the high attenuation of a single cor- 
ner, propagation around multiple corners 
is even more severely attenuated. 

Although it is an advantage to oper- 
ate at a higher frequency in a straight 
tunnel, the higher frequencies suffer 
the greatest loss in turning a corner. 
Therefore, the choice of frequency is 
often dictated by the type of coverage 
desired. 

Based on the interference and signal 
attenuation rates observed, the effective 
communication range for UHF radios can be 
predicted. Figure 3-33 shows the pre- 
dicted range for a 1,000-MHz, 1-watt 
portable transceiver. 

The presence of stoppings for direc- 
tion of airflow, passages blocked by 
machinery, or blockage caused by a roof 
fall seriously limits the communication 
range of a UHF system. Obstructions 
highly attenuate all UHF signal transfer, 
thus making the same systems impractical 
for some mine applications. 



3300 FT 



XDOL 



JJLL 



pjcm 



UJJJDODnnpZLLi 



3300FT -.ii^ 



LZiuuuuun^nGMuu! iL_\_} 

— innnnnnrTTininnnn rnn — 
rpTT nnnn rtT"^ i • ' ' 
i I I i i n.nrr-i 1 i i i 



JJC£ 



TjCL 



ES 



jnrr 



a 



i I I M I n-rr 



} \ i 1 I I I I I 1 I I t L 

i 1 I I I I J J I V 1 ; 1 I I ; J 



FIGURE 3-33. - Predicted range for 1,000-MHz, 
1-watt portable transceiver. 



64 



3.5.3 Belt Haulage 

Mines using conveyor belts to move 
coal or ore underground usually have a 
secondary transportation system for the 
movement of men and materials. When the 
man-material transportation system is 
tracked-trolley , the obvious solution to 
haulageway communication requirements is 
the trolley phone system described in 
section 3.5.1. If the man-material 
transportation system is nontrolley, then 
some form of radio, leaky feeder, or 
wired phone system is required. 

A coimnon practice in mines using 
belt haulage is to locate telephones at 
the intersections of all mains and sub- 
mains, and at the head and tail of all 
working conveyor belts. (Belt fires most 
often occur at these points.) In the 
absence of trolley phones, belt haulage 
mines also usually locate phones approxi- 
mately every 600 feet along the belts. 
These phones are installed for the life 
of the mine and are seldom moved. 

Although a fully developed submain 
might have butt entry ports every 
600 feet along its length, telephones are 
required only at the active or working 
butt entry ports. This usually limits 
the maximum number of phones per submain 
to six, owing to the capacity of most 
haulage systems. These phones are moved 
about every year or so until all panels 
in the submain have been developed. If a 
feeder belt is used in the submain, addi- 
tional phones are recommended at the head 
and tail of these belts. 

Phones permanently installed at the 
head and tail, and at other strategic 
locations along the belt, usually meet 
the communication requirements during 
normal day-to-day operations. The draw- 
back to any wired phone system is that a 
miner must be at a phone to make or 
receive a call. Communication with belt 
maintenance or inspection personnel mov- 
ing along the haulageway can only be 
accomplished by some form of radio link. 



The same systems described in sec- 
tion 3.5.2 (nontrolley haulage) can be 
utilized to meet the communication 
requirements in belt haulageways. 

3.6 Special Requirements 

This section describes ways of meet- 
ing those special communication require- 
ments not directly related to the mine 
entrance (section 3.2), permanent and 
semipermanent locations (section 3.3), 
mining areas (section 3.4), and haulage- 
ways (section 3.5). Major topics 
included in this area of special require- 
ments include communications with roving 
or isolated personnel and motorman- 
to-snapper communications. 

3.6.1 The Roving or Isolated Miner 

A modern mine is a vast underground 
complex of working sections, haulageways, 
and repair shops , which extends for sev- 
eral square miles underground. Key per- 
sonnel may not work in fixed locations; 
for instance, a section foreman may be 
assigned to a single section, but that 
section could embrace a vast area, or 
maintenance personnel or electricians 
could be anywhere in the mine at any 
time. Because such personnel are impor- 
tant to the smooth operation and high 
productivity of a mine, considerable pro- 
duction losses can occur if they cannot 
be located when they are needed. 

Inspectors and other management per- 
sonnel may also be anywhere in the under- 
ground complex. These people need to 
stay in continuous contact with the com- 
munication center so that they can be 
informed of any emergencies that might 
arise and/or make management decisions. 

The maintenance crew is also spread 
throughout the mine. To receive repair 
requests and dispatch his crews for emer- 
gency or nonscheduled repair work, a 
maintenance foreman must be able to con- 
tact individual crew members dispersed 
throughout the mine. 



65 



Communication requirements to and 
from these key individuals can only be 
completely satisfied by a wireless 
(radio) paging or walkie-talkie system. 
Several paging systems are presently 
available to meet these requirements. 
The small lightweight pagers that can be 
carried by roving personnel are classi- 
fied as one of three types: 

Beepers (call alert). 

One-way-voice (pocket pagers). 

Two-way-voice (walkie-talkies). 

One shortcoming of the first two 
types of systems (beepers and pocket 
pagers) is that the person initiating the 
page has no way of knowing if the page 
has been received. This can be espe- 
cially critical in the case of the pocket 
pager systems where voice messages can be 
transmitted to the person being paged. 
Because the pocket pager is a receive- 
only device, the person being paged can- 
not directly notify the dispatcher or 
person making the page that he has 
received the message. Therefore, one- 
way-voice (pocket) pagers should only be 
used for paging messages ("call the dis- 
patcher," "report to the maintenance 
area," etc.). Instructions such as "shut 
off the number 2 pump" should not be 
given using one-way communication devices 
unless it can be verified that the mes- 
sage was received and acted upon. The 
advantages gained by any of the three 
types of paging systems are directly 
related to the reduced time required to 
contact key individuals when 
tion underground is unknown, 
the simplest beeper systems , 
being paged can, within a few seconds, be 
headed for a section phone to take a 
message. 

3.6.1a One-Way-Voice (Pocket) Pagers 

In a mine that uses rail haulage 
vehicles powered from an overhead trolley 
wire locomotive or jeep, carrier phones 
allow the vehicle operators to com- 
municate with each other and with a 



their loca- 

Even with 

the person 



dispatcher who controls the flow of 
traffic. As explained in section 2.4, 
the trolley line itself is the communi- 
cation link between all the vehicles and 
the dispatcher. 

However, communication need not be 
limited to phones connected to the 
trolley line. A special carrier-current 
tone signal can also be impressed on the 
trolley line, which will function as a 
long-line antenna, broadcasting the tone 
signal into the mine where it can be 
received by special pocket 
(fig. 3-34). Hardware is 
cially available that allows a dispatcher 
to voice-page selected individuals , 
deliver short messages, or inform them 
where to go to receive detailed instruc- 
tions. Figure 3-35 is a block diagram of 
a general radio paging system based on 
carrier-current techniques. A carrier 
phone, located at some central loction 
such as a dispatcher's room, is equipped 
with a small pushbutton-encoder unit. 
This unit causes the carrier phone to 
transmit short tone bursts whose frequen- 
cy depends on which pushbutton was 
pushed. These tone bursts are transmit- 
ted from the carrier phone in exactly the 
same way that a voice signal would be 
sent out. 



radio pagers 
now commer- 




FIGURE 3-34. 
radiopager. 



Miner equipped with pocket 



66 









TROLLEY LINE 


II DOES NOT TURN 
ON 2 




r • 


SELECTIVE PAGING 
SIGNALS TO PAGER 1 






CARRIER 
PHONE 
















BEEP PLUS \ 

VOICE 

MESSAGE ^ 


mil 






MANUAL 
PUSH BUTTON 


POCKET PAGER 
I TURNS ON 




ENCC 


DED 







DISPATCHER'S OFFICE 

FIGURE 3-35. - Block diagramof general radio- 
paging system. 

The pocket receivers that have been 
developed to respond to these tones are 
really small FM radio receivers that are 
activated by the tones and remain on for 
about 15 seconds. Once the tones have 
been sent, the dispatcher then talks into 
his carrier phone in the usual manner. 
Only the pocket pager activated by the 
tones will receive the message, so that 
the dispatcher can selectively radio-page 
any individual. In an emergency, a spe- 
cial tone can activate all pagers at 
once. The pocket pager is a receiver 
only and cannot be used to talk back to 
the dispatcher. Therefore, the system 
should be used only for paging, not for 
giving instructions. 

The system shown in figure 3-35 is 
designed so that only the dispatcher can 
initiate a page, because he is the only 
one who has a carrier phone equipped with 
an encoder. However, other encoders 
could be used with other carrier phones, 
if necessary. Figure 3-36 shows a system 
in which the encoder is remotely accessed 
by a dial telephone line. Thus, any dial 
telephone associated with the mine 
switchboard (PBX) could be used to initi- 
ate a page without ever being near the 
encoder. Such a system offers an advan- 
tage should many people have to page into 
the mine from several surface locations. 
To operate the system, a user goes to a 
telephone and dials the number assigned 
to the pager he or she wishes to call. 
The encoder converts the telephone dial 
pulses into tones and transmits them via 
the carrier phone. The tones turn on the 



eo3 — ^ 
c°3 



///////////////////// 



UNDERGROUND 



TROLLEY LINE 



V77/ 



>\ 



1 1 




M 



/^ 



FIGURE 3-36. - Block diagram of system with 
remotely accessed encoder. 

desired pager, at which time the user can 
speak into the mouthpiece to deliver the 
voice message. 

Existing pager receivers are 
equipped with a small internal timer that 
automatically turns the device off after 
a preselected time, usually 15 seconds. 
A continuous "On" mode is usually not de- 
sirable because it wastes battery power. 
With the automatic time-out feature, bat- 
teries last for months. However, there 
are times when the continuous monitoring 
of the radio paging system is important 
to certain maintenance personnel. 

A radio paging system can be oper- 
ated on a special channel (frequency), 
or on the regular channel used by the 
locomotives. The only difference is that 
if both are included on the same regular 
channel, all the carrier phones will 
hear the paging traffic, but the pagers 
will hear only what is sent to them 
directly. 

A radio paging system can incor- 
porate both the automatic encoded system 
(fig. 3-36) and a roof-bolt antenna sys- 
tem (fig. 3-37). The automatic encoder 
and carrier phone can be located on the 
surface; all else is underground. The 
in-mine roof bolts are separated by about 
300 feet and connected to the carrier 



67 



POCKET PAGER 







MOTOnwAN 



TROLLEY LINE 
(GIVES HAULAGEWAY COVERAGE) 



(GIVES REMOTE COVERAGE) 







11 

I POCKET PAGER 



CARRIER 

PHONE 
REPEATER 



III II 



POCKET PAGERS 



FIGURE 3-37. - Block diagram of system using 
roof-bolt-type antenna. 

phone by No. 12 wire. With this system, 
paging can be accomplished from as far as 
500 feet from a roof bolt antenna. 

3.6.2 Motorman-to-Snapper 

Many mines use loading track loops 
for loading mined coal or ore at the sec- 
tions before transporting it to the sur- 
face. This type of operation involves 
the coordinated activities of two indi- 
viduals: a "snapper" or "swamper" who 
couples and uncouples the cars; and a lo- 
comotive operator, or motorman, who moves 
the train backward and forward at pre- 
scribed times. If the snapper is not in 
the clear when the train is moved, its 
sudden motion can injure or kill him. 
Thus, effective communication between the 
motorman and snapper is vital. 

Because of the curvature of the loop 
track (fig. 3-38) and the location of the 
locomotive on the main haulage track, the 
two individuals are not usually within 
sight or hearing of each other. Without 
communication or at least some system of 
signaling, coordination is difficult un- 
less other workers are stationed along- 
side the track to relay information. 
However, this wastes time and manpower. 
It is clear that the safety and efficien- 
cy of the loading operation would be 
vastly improved if there were a reliable 
communication link between the motorman 
and snapper. 




DECOUPLES CARS 



FIGURE 3-38. - Diagram showing need for com- 
munication in haulage loop-around. 

The design of any practical system to 
meet the communication needs between mo- 
torman and snapper requires that it does 
not interfere with other communications, 
is convenient, has a restricted range so 
that similar systems can be used else- 
where in the mine, and can be built with 
commercially available hardware. Typi- 
cally, a range of 1,500 feet or less is 
all that is necessary to assure adequate 
coverage for the maximum separation be- 
tween the snapper and motorman. Two sys- 
tems that can presently be implemented 
using commmercially available hardware 
are the telephone and trolley-carrier 
phone system and the walkie-talkie radio 
system. 

3.6.2a Telephone and Trolley-Carrier 
Phone System 

In the telephone and trolley-carrier 
phone system (fig. 3-39), the snapper 
communicates by means of a belt-carried, 

TROLLEYLINE 




EXTENSION CORD 



FIGURE 3-39, - Telephone and trolley-carrier 
phone system. 



68 



miniature mine telephone known as a belt 
phone. A phone line is installed on the 
rib or roof of the mine along one side of 
the loading track. The belt phone can be 
connected to this line by an extension 
cord that has insulation-piercing clips 
at one end. Alternatively, receptacles 
that allow the belt phone to be plugged 
in at convenient points can be provided 
on the line. 

A pager phone to carrier phone cou- 
pler connects the phone line and the 
trolley line. Phone line signals are 
converted to trolley line signals and 
vice versa by this coupler. The motorman 
communicates by a trolley phone, which 
operates on a frequency different from 
that of the haulage communications. 



CAUTION.— It should also be noted 
that phones connected to the trolley 
line in this manner are not permissi- 
ble, and should be separate from the 
main phone system. Any such inter- 
connect musts be coordinated with 
MSHA. 



If duplicate systems are used in a 
mine, the range of the trolley line sig- 
nals has to be restricted by appropriate- 
ly attenuating the transmitter output. 
This system can be implemented using 
standard trolley phones and phone-line- 
to-trolley-line couplers. In addition, a 
belt phone (fig. 3-40) is now commercial- 
ly available. Equipped with a hardhat- 
mounted speaker and an adjustable-boom- 
type microphone, it has outgoing paging 
capability and will operate compatibly 
with available phone-line-to-trolley-line 
couplers. 

At least one mine has successfully 
used an interface system between the 
phone and trolley lines to provide 
motorman-snapper communication. The sys- 
tem is diagrammed in figure 3-41. A re- 
mote interface, fabricated by technicians 
at the mine, acts as a coupler between 
the trolley line and a dedicated phone 
line. The motorman can communicate via 
the existing carrier phone system, where- 
as the snapper mast communicate via 




FIGURE 3-40. = Miner wearing belt phone„ 



CARRIER PHONE 
'^ ON MOTOR 


TROLLEY 


LINE 






^^/ 


PHONELINE 






1 MOTOR 1 




O U 




FH ) 


I 






^ l^ 


INTERFACE 






rVs. vV> 







^JACKBOXES 



FIGURE 3-41. - Diagram of interface system 
between phone and trolley lines. 

the phone line using a modified telephone 
handset. A twisted-pair phone line, with 
jackboxes connected at 50-foot intervals, 
is strung up in the loop-track area and 
connects to the dedicated phone line. 
The snapper plugs his handset into a 
nearby jackbox to establish communication 
to the motorman. 

3.6.2b Walkie-Talkie System 

The walkie-talkie radio system uses 
UHF portable radio equipment. Both the 
motorman and snapper are equipped with 
walkie-talkies (fig. 3-42). 



69 




FIGURE 3-42. - Motorman and snapper walkie-talkie 
system. 

Because of the curvature of the loop 
tunnel, propagation of radio waves at UHF 
is severely restricted. In fact, direct 
radio communication between the two in- 
dividuals may not be possible in some 
cases. However, this deficiency can be 
overcome with a dual-frequency radio re- 
peater connected to a radiating cable. 
The cable carries the radio signals, and 
the repeater effectively boosts them to 
a higher power level. The coaxial cable 
extends along the loading track and down 
the main haulageway far enough to assure 
communication coverage to the motorman. 
Cables several hundred feet shorter can 
be used if an approprate antenna is con- 
nected at the end. Commercially availa- 
ble portable radio transceivers and re- 
peaters can be used to implement this 
system. 

A medium-frequency radio transceiver 
(520 kHz) with sufficient range has been 
developed that makes snapper-motorman 
communications possible 
ing additional cables, 
aided by the conductors 
in the loop-around. 



without install- 

Transmission is 

normally present 



Effective communication between the 
snapper and motorman can provide 
the coordination needed to eliminate 



uncertainties regarding train movement in 
the mine. This results in improved effi- 
ciency and a reduction in the num-ber of 
accidents related to the loading opera- 
tion. Systems can be custom made from 
available telephone and carrier phone 
equipment. Leaky-feeder UHF equipment is 
similarly available for custom systems. 

3.7 Emergency Communications 

There are two conditions under which 
a communication system should operate. 
These are normal operations (regular day- 
to-day operation) and emergency condi- 
tions. The need for reliable underground 
communications following a disaster is 
obvious. Two major requirements for any 
emergency communication system follow: 

1. The system must work following 
the disaster. (This implies that the 
system worked before the disaster and 
that the system is protected from, or 
immune to, fire, explosion, roof fall, 
etc. ) 

2. Miners must be familiar with 
operation of the system. (Mistakes are 
easy to make during periods of high emo- 
tional stress.) 

It should be recognized that there 
are advantages in combining any emergency 
communication system into the system used 
for normal day-to-day operations. In 
this way, miners can become familiar 
enough with the system to operate it dur- 
ing disaster conditions. Daily use of 
the system also provides a mechanism of 
regular testing, thus insuring that the 
system will be operational. 

3.7.1 Detecting and Locating the Trapped 
Miner 

The history of coal mine disasters 
has established a need for a simple, 
reliable system for locating and communi- 
cating with miners trapped underground. 
Such a system will not only increase the 
chances of a successful rescue, but will 
also reduce the risks to the rescue team 
by keeping them from searching the wrong 
locations. 



70 



The problems of finding miners 
trapped underground can be illustrated by 
a disaster that occurred in 1945, in 
which 24 men were killed by an explosion. 
Figure 3-43 shows the location where nine 
men barricaded themselves for 53 hours in 
that particular incident. Rescue crews 
tried for 2 days to reach the active area 
of the mine in 5 and 6 Lefts while being 
hampered by caved workings, fires, smoke, 
gas, and loose roof. Three days later, 
while exploring 9 Right, they found foot- 
prints. After investigating, they found 
a chalk-marked board indicating that five 
men were in 4 Left entry. In 5 Left, 
another mark was found directing search- 
ers to second Left off of 5 Left. Seven 
of the nine men survived the ordeal. All 
might have lived if their location had 
been known so they could have been 
reached sooner. The time required to 
rescue barricaded miners is critical. In 
the recorded cases of barricading, 75 
percent of the survivors were rescued 
within 10 hours. 




'I I 

UthVJ 

"< ^ <;' t. ^ Cef Booster fan j jj ) 

!>9M., '■■,-;-.o..v->,. \ :,■ , , 



7 2lxxlln 




After a disaster, miners who manage 
to escape can direct rescue teams to 
those parts of the mine where others may 
remain trapped. The nature of the mine 
workings and the circumstances of the 
disaster can also be used in locating 
survivors, but all of these techniques 
are based on guesswork. Accurate knowl- 
edge of the location of trapped men is 
required to increase their chances for 
survival and to reduce the hazards to the 
rescue team that might otherwise conduct 
an unnecessary, futile search in danger- 
ous, incorrect areas. 

It is obvious that any information 
that could be exchanged between the 
trapped miners and the rescuers during a 
rescue effort would be advantageous. In- 
formation such as unusual conditions 
known to the miners trapped, 
advice for them to follow 
arrived, are two examples, 
words , a system that would 
location of trapped miners 
communication with them would 
the probability of their rescue 



FIGURE 3-43. - Part of mine showing areato which 
miners retreated and erected imperfect barricade. 



or medical 

until aid 

In other 

provide the 

and permit 

increase 

and also 

reduce hazards to the rescue and recov- 
ery team. Two systems for locating and 
communicating with trapped miners have 
been developed: a seismic system and an 
electromagnetic system. 

The seismic system relies on detec- 
tion of small ground vibrations resulting 
from a miner(s) banging on the roof or 
ribs with some heavy object. This system 
is presently operational and is being im- 
proved continuously. In this system, the 
trapped miner signals on the mine floor 
or roof with any heavy object and seismic 
detectors (geophones) on the surface are 
used to detect these signals. Computa- 
tion of the location of the trapped miner 
by using the difference in the arrival 
time of the signals at various geophone 
positions on the surface has been quite 
successful. A seismic location system 
has the advantage that the miners do 
not require any special equipment and 
need only to be trained in how and when 
to signal. The disadvantage is that 
discontinuities in the overburden can 
significantly affect rescue signal propa- 
gation relative to both detection and 
computation of location of the signal. 



71 



Additionally, in a rescue and recovery 
operation, the time required to deploy 
and relocate, if necessary, a massive 
geophone array may hamper the progress 
desired. However, the seismic system 
does provide the trapped miner with an 
additional degree of protection when no 
other method of communication can be es- 
tablished. The Mine Safety and Health 
Administration maintains a seismic rescue 
system as part of its Mine Emergency 
Operations group. All miners should ob- 
tain MSHA stickers for their hard hats 
(fig. 2-26) in case they should become 
entrapped. 

The electromagnetic system relies on 
a small voice frequency (VF) transmitter 
that can be carried by the miner, and 
surface receivers that "listen" for the 
signals broadcast directly through the 
earth or through the mine workings by the 
miner's transmitter. Basic development 
of VF EM systems is completed, and proto- 
type hardware is in the testing phase. 

A typical trapped-miner transmitter 
(fig. 3-44) weighs one-half pound and can 
be worn on the belt. Cap lamp battery 






units also exist. In an emergency, 
and when it is decided that all routes of 
escape are closed, the antenna wire is 
uncoiled, laid out in as large a loop as 
possible, and connected to the trans- 
mitter. The transmitter and loop antenna 
produce a magnetic field, as shown in 
figure 3-45. The direction of these 
signal-field lines can be used to pin- 
point the location of the underground 
loop antenna. By measurements taken on 
the surface, the location of the antenna 
can be determined within a few feet. 

After detecting and locating a 
trapped miner, the surface search team 
can establish a voice down-link communi- 
cations path to the men underground. 
This voice link is established by deploy- 
ing a large loop antenna directly above 
the trapped miners and connected it to a 
very powerful amplifier and voice system 
(fig. 3-46). By speaking into the micro- 
phone associated with the system, strong 
electromagnetic signals are generated and 
transmitted by the loop antenna. These 
signals penetrate the earth, and the 
trapped miners can hear actual voice from 
the surface on their transceiver. The 
surface can then ask key questions to 




MAGNETIC FIELD LINES 
(VERTICAL DIRECTLY 
OVER LOOP) 



VF TRANSMITTER 



FIGURE 3=44. = Underground=miner=carried VF 
equipment for signaling surface rescue crew. 



II 1 \ 

FIGURE 3-45. - Production of a magnetic field 
by transmitter and loop antenna. 



72 



^' 



LOOP ANT EN 




'^^^>N [~^;^r:^7"[ x::z::> 



^^;;j;;j5.;;5^^?5755^^^,,^^^ 



LOCATION 
AND CODE 
SIGNALS 



A 




N 



FIGURE 3-46. 
system. 



Through=the-earth transmission 



ascertain the conditions underground. As 
an example, they can ask the trapped 
miners to key three pulses of signal for 
a "yes" answer and two pulses for a "no" 
answer. This type of down-link voice and 
up-link code signaling system allows the 
surface team to learn anything they wish 
about the situation underground and also 
allows them to give instructions or 
information concerning escape routes and 
rescue attempts. 

One advantage of an electromagnetic 
system over a seismic system is that 
the EM transmitter operates continuously 
once deployed and will function for many 
hours, or even days, from one cap lamp 
battery. Besides operating continuous- 
ly, its electrical signal is a known 
rhythmic "beep," which is much easier 
to detect than the random thumps of a 
miner pounding on the ribs or roof. An- 
other advantage is that the detection re- 
ceiver can be readily carried by one min- 
er (fig. 3-47) and can be used to cover a 
reasonably large area. It can also be 
used by underground rescue teams since it 
is permissible. A version of the surface 
receiver has been adapted for use in hel- 
icopters. With this unit, large areas 
can be scanned quickly. Once a signal is 
detected, portable surface-carried units 
can obtain an exact fix. The surface 
gear for a seismic system, on the other 
hand, is complex and stationary. Its de- 
ployment site must be carefully selected. 
If it is not within 2,000 feet of the 




FIGURE 3-47. - Surface VF receiver and loop 
antenna in use at simulated mine disaster. 

signal source, it probably will not work. 
In a large mine, this limitation is a se- 
rious handicap. In mountainous terrain, 
setting up the seismic geophones can pre- 
sent especially difficult problems. 

3.7.2 Refuge Shelter 

When it appears to be impossible to 
escape, or imprudent to attempt escape, 
following a mine fire or explosion, 
miners are trained to isolate themselves 
from toxic gases and smoke by erecting 
barricades. Although many miners have 
been rescued from behind barricades, some 
have died behind inadequately constructed 
barricades. As a solution to this prob- 
lem, sectional or central refuge chambers 
have been established by some companies. 
If a chamber is constructed, some form of 
communication to the surface should be 
included to inform rescue crews that the 
chamber is being used and of the condi- 
tion of its occupants. 

Communication to a refuge shelter 
could be provided by means of a borehole 
equipped with a telephone pair connect- 
ing to the surface, by existing wiring 
within the mine, or by some form of 
through-the-earth system. The in-mine 
telephone system would be the least reli- 
able after an explosion unless the cable 



73 



installation had been specifically hard- 
ened. Boreholes would be highly reliable 
but would require a new borehole for each 
refuge chamber or whenever a refuge cham- 
ber was moved. 

3.7.3 Rescue Team Communications 

Even though searching a mine after a 
fire or explosion is a slow and often 
dangerous job, the rescue team must reach 
any trapped or barricaded miners as soon 
as possible. Effective communication 
between the rescue team and the surface 
or base camp, as well as communication 
between individual members of the team, 
is an essential element in any successful 
rescue attempt. 



The primary advantage of this type 
of system is that it is simple and yet 
usually provides good-quality voice com- 
munication. Also the phone wire trailed 
behind the rescue team provides a physi- 
cal link back out of the search area. 
This link can become an important factor 
if the team must retreat under conditions 
of poor visibility, or if a second rescue 
team wishes to "follow" the first team. 
The disadvantages of this type of system 
are (1) the wire spool, which may be 
heavy, must be transported by the rescue 
team, and (2) the wire strung behind the 
rescue team is susceptible to damage from 
secondary explosions or roof falls. 

3.7.4 Medium -Frequency Rescue Systems 



One method that has proven effective 
in maintaining communication to and from 
the rescue team is illustrated in figure 
3-48. In this relatively simple system 
the rescue team splices into a good phone 
line and then unrolls line from a spool 
as they advance into the mine. During 
a recent rescue, this type of system 
provided good communication even after 
the rescue team had traveled approximate- 
ly a mile along a haulageway and then 
descended another 1,200 feet down a shaft 
from an underground headframe. 



PERMANENT PHONE LINE 




PHONE LINE 
WOUND 
ON SPOOL 



FIGURE 3-48. - Effective method for maintain- 
ing communication to and from rescue team. 



Considerable research has been con- 
ducted within the last 8 years in the ar- 
ea of underground MF transmissions. This 
research showed that MF signals could 
propagate for great distances in most ge- 
ologies and offered the hope of a whole- 
mine radio system. The Bureau of Mines 
and the South African Chamber of Mines 
(SACM) pursued research independently. 

Around 1974, SACM introduced a new 
single-sideband system and followed up 
later with another designed especially 
for rescue team use. Performance in 
South Africa was reported to be good. 
The evaluation of these units in U.S. 
mines showed them to be inadequate. The 
type of modulation used [single sideband 
(SSB)] made them sensitive to electromag- 
netic interference (EMI). In addition, 
power level was far too low and ineffi- 
ciencies in both circuit and antenna de- 
signs produced short-range performance. 

The Bureau's approach to the problem 
was more fundamental. A program was de- 
signed and executed to study in-mine MF 
propagation and learn how it interacted 
with the complex environment. This envi- 
ronment consists of various geological 
factors such as stratified layers of dif- 
ferent electrical parameters, entry size, 
local conductors, EMI, etc. 



74 



Figure 3-49 is a simplified geometry 
of an in-mine site that illustrates one 
of the most important findings of the 
measurement program — the "coal seam 
mode." For this mode to exist, the coal 
seam conductivity (0c) must be several 
orders of magnitude less than that of the 
rock (Op). A loop antenna that is at 
least partially vertically oriented, pro- 
duces a vertical electric field (E^) and 
horizontal magnetic field (H<j)). In the 
rock, the fields diminish exponentially 
in the Z-direction. In the coal seam, 
the fields diminish exponentially at a 
rate deteirmined by the attenuation con- 
stant (a) which in turn depends upon the 
electrical properties of the coal. An 
inverse square-foot factor also exists 
because of spreading. The effect is that 
the wave, to a large degree, is trapped 
between the highly conducting rock layers 
and propagates long distances within the 
lower conducting coal seam. The fact 
that the coal may have entries and cross- 
cuts is of minor consequence. 

In the presence of conductors, the 
picture changes considerably. In this 
case, the effects of these conductors can 
totally dominate over the effects of the 
geology. In general, the presence of 
conductors (rails, trolley lines, phone 
lines) is advantageous. 

MF signals can couple into, and re- 
radiate from, continuous conductors in 
such a way that these conductors become 
not only the transmission medium but 
also the antenna system for the signals. 
Figure 3-50 illustrates this concept. 
The most favorable frequency depends to 
some extent on the relationship between 
the geology and existing conductors. 
The frequency effects are quite broad. 
Anything from 400 to 800 kHz is usually 
adequate. 

3.7.4a Specific Application of MF 

Communications to Rescue Teams 

The low attenuation of MF signals in 
many stratified geologies, such as coal 
mines, can be of great benefit to rescue 
teams. If existing mine wiring (like 





^c 



Coal 

or 

entry 



Loop 

transmitting 

antenna 



FIGURE 3-49. - Coal seom mode. 



Local mine conductor 




Signal reradiating 
from conductor 



FIGURE 3-50. - MF parasitic coupling and 
reradiation. 

powerlines or belt lines) are present, 
the range is even greater. This permits 
a rescue team member to stay in commu- 
nication with other members, the fresh 
air base, and outside disaster control 
centers. 

To date, MF technology has not been 
specifically applied to rescue team 
communications. Such application is the 
second step in the Bureau's overall MF 
communications program. However, there 
is no basic difference between opera- 
tional MF systems and postdisaster MF 
systems. By October 1982, the Bureau's 
operational MF systems was in place in 
several cooperating underground mines. 
By October 1983, performance evaluation 
of the systems will be completed. As the 
performance proceeds, emphasis will be 
directed to specific postdisaster-rescue 
applications. 



75 



3.7.4b System Concepts 

The main advantage of MF communica- 
tion is simplicity. Figure 3-51 shows a 
rescue team member equipped with a pro- 
totype MF vest radio. This vest radio 
permits rescue team members to maintain 
local communications (fig. 3-52). 

In most cases, rescue teams will 
utilize a lifeline for rapid retreat in 
case of smoke when visibility is limited. 
The lifeline offers interesting possibil- 
ities for MF radio communications. Some 
rescue teams actually use the line al- 
ready to carry communications via sound- 
powered telephones. Such a scheme is 
both archaic and often ineffective. 

Since this line is a continuous con- 
ductor back to the fresh air base, it 
provides a convenient parasitic path for 
MF communication as shown in figure 3-53. 
To assure even more reliable communica- 
tions, physical audio links could be made 
with the lifeline as shown in figure 3- 
54. Such an approach provides redundancy 
via simultaneous audio and radio links. 

Figure 3-55 illustrates a total MF 
base station for rescue team use. At the 
fresh air base, the briefing officer (as 
the individual is sometimes called) is 
equipped with a standard intrinsically 
safe base station or repeater; the offi- 
cer could also be equipped with a vest. 
With such an arrangement, communications 
are possible not only between rescue team 
members, but also with the surface and 
with other distant rescue teams. In ad- 
dition, it also provides a possible link 
to the trapped miners. 

Since existing mine wiring is exten- 
sive and minewide, it is easily seen that 
it provides yet another redundant link 
for the rescue team members. Since other 
rescue teams are also in the vicinity of 
mine wiring, interteam communications are 
possible if desired. This concept of in- 
terteam communications is a radical de- 
parture from existing procedures. It 
will permit one rescue team, in one part 
of the mine, to modify the ventilation in 
such a manner that it does not degrade 




FIGURE 3-51. = Rescue team member with 
MF vest radio. 




FIGURE 3-52. - Basic MF communications 
among rescue team members. 



76 



Life line and/or local mine conductors 



Vest |_v 
radio 



Vest 
radio 



Advancing 
rescue team 



Fresh 
air base 



FIGURE 3-53. • LifelineasaparasiticMF path. 



Plug 



I Audio link 



Audio plus 
MF 



Vest 
radio 



^f> 



Y 



Vest 
radio 



team communications 



Fresh air 
base 



FIGURE 3-54. - Lifeline as a redundant commu- 
nications line for MF and audio communication. 



Local mine wiring 



Life line 



Base 
station or 
repeater 



Signal coupler 



Microphone 



FIGURE 3-55. - Total MF base station for 
rescue teams. 

the ventilation in the vicinity of an- 
other rescue team. Equally important is 
the fact that trapped miners are also 
probably in the vicinity of existing mine 
wiring. 

3.7.4c Location and Communications 
Systems for the Rescue of 
Trapped Miners 

So far this section has primarily 
addressed the application of MF com- 
munication to rescue teams. However, 
the ultimate objective of the rescue 



operation is to reach trapped miners in a 
timely manner before they succumb to the 
effects of injury, exposure, or toxic at- 
mospheres. To this end, rescue team com- 
munications is but a part. The key to 
successful rescue lies in the rapid loca- 
tion of the trapped miners. Without 
this, valuable time can be wasted in 
diverting rescue efforts to the wrong 
area, often with tragic results. 

Bureau research in the area of lo- 
cation has been addressed by through- 
the-earth seismic and EM systems. In 
the seismic system, trapped miners pound 
on the roof or ribs of the mine to gen- 
erate seismic vibrations. These vibra- 
tions travel through the overburden to 
the surface where they can be detected 
by sensitive transducers called geo- 
phones. Computer analysis of the arrival 
times of the seismic signals at the 
various geophones permits the source to 
be accurately located. This system is 
operational and is kept in readiness by 
MSHA Mine Emergency Operations. Present 
Bureau research in EM means to locate 
and communicate with trapped miners is 
shown in figure 3-56. The system con- 
sists of two parts, a transceiver that 
is normally carried on the miner's belt 
and a surface system for detection and 
communications . 



Loop antennas 



rf^ ^ K b 




/Microphone 



Transmitter 



Location signal 

A 



Transceiver 

Battery with 

power 
take-off 

FIGURE 3-56. - Voice frequency electromag- 
netic system for location and communication 
with trapped miners. 




77 



In operation, the trapped miner re- 
moves the transceiver from the belt, de- 
ploys a self-contained loop antenna, and 
attaches the transceiver to a special cap 
lamp battery. This antenna consists of 
300 feet of No. 18 wire that must be de- 
ployed in the largest area possible to be 
effective. A location signal is trans- 
mitted directly through the earth. 

On the surface, sensitive receivers 
detect the signal and locate the source. 
Once detection and location are made, a 
large surface transmitter is deployed 
above the trapped miner. This transmit- 
ter is powerful enough to send voice mes- 
sages by radio, directly down through the 
earth. 

The trapped miner's transceiver re- 
ceives this voice. The surface personnel 
then ask the miner "yes-no" questions 
concerning his or her condition and that 
of the mine. The miner responds by sim- 
ple on-off keying of the transceiver. In 
this manner a two-way communications link 
is established, entirely through the 
earth, and rescue operations can start in 
the most efficient manner. 

Details of this EM system can be 
found in numerous reports. This is known 
as a voice frequency (VF) system because 
all communications take place in the VF 
band of 300 to 3,000 Hz. 

The seismic system is very effective 
in mines up to 2,200 feet deep, and does 
not require the miner to be equipped with 
any special devices. However, it does 
require the miner to be able to pound. 
Injury or lack of a sufficiently heavy 
object with which to pound may render the 
system ineffective. The most serious 
drawback is that of time. The surface 
receiver station (geophones, field truck 
with computer, etc.) may take too long 
to set up. Bad weather and terrain can 
further delay the surface station deploy- 
ment. 

The EM-VF receiver system is less 
affected by adverse conditions on the 
surface because it is lighter and more 
easily transportable. However, it has 



its own disadvantages. The trapped miner 
must be equipped with a special trans- 
ceiver, and must be able to deploy the 
antenna in a sufficiently large area. 
Injury or confined quarters may prevent 
deployment. In addition, under the best 
of conditions, the system has a range 
limit of about 1,000 feet. Although a 
new system is under development that will 
increase the range to 3,000 feet, this 
improvement comes about only with com- 
plex, slowly deployed surface equipment. 
Therefore, it will be subject to the same 
delays as the seismic system. 

MF communication offers advantages 
over through-the-earth approaches by 
permitting in-mlne communications to 
the trapped miners. This could be in 
addition to, or in place of, through- 
the-earth schemes that may fail because 
of excessive overburden or the inability 
of the trapped miner to deploy his or her 
end of the system successfully. Fig- 
ure 3-57 illustrates this concept. 

In this illustration, the trapped 
miner is equipped with a small MF trans- 
ceiver built into the top of the cap lamp 
battery or worn on the belt. Note that 
this is exactly the same packaging con- 
cept used for the VF through-the-earth 
system shown in figure 3-56. The intent, 
however, is not to send a signal through 
the earth, but rather to induce a signal 
onto local mine wiring. If this is ac- 
complished, the in-mine rescue team also 



(A C power line) 
Local conductor No. 2 



(Trolley line) 
Local conductor No I 



Life line 



( Parasitic ] 
I coupling I 



Base station 
or repeater 



J 



Trapped miner 
MF transceiver 



Vest 
transceiver 



Distance can be miles 



-»■ [Trapped minerj 



[Rescue team] 

FIGURE 3-57. - MF in-mine location and com- 
munication system. 



78 



is likely to be in the vicinity of mine 
wiring and can receive the signal. It 
must be pointed out very clearly that 
mine wiring does not mean that one con- 
tinuous assembly of wiring is involved. 
If the trapped miner is near a power ca- 
ble and not near a trolley line, and the 
rescue team is near a trolley line and 
not near a power cable, this does not 
mean that a communications link between 
the two cannot exist. An induced MF sig- 
nal on one type of conductor will para- 
sitically couple to all others, even if 
there is no physical connection. This is 
the uniqueness of MF communication. 



signals of narrow bandwidth that parasit- 
ically couple onto mine wiring, and are 
widely distributed. This can be received 
by the in-mine rescue team. If this oc- 
curs, they will use their more powerful 
MF equipment (vests or base stations) to 
establish a voice link to the trapped 
miner. By asking the trapped miner yes 
or no questions, his or her location can 
be learned. However, direct location via 
MF communication is impossible. The par- 
asitic coupling characteristics of MF 
signals do not permit the through- 
the-earth VF type of location; the signal 
could be on many conductors. 



In operation, the trapped miner Obviously VF and MF systems could be 

would deploy an MF loop antenna or cou- combined such that the benefits of both 

pier, preferably onto available local VF (fig. 3-56) and MF (fig. 3-57) could 

wiring. The coupler could be a small de- be obtained. Equally important is the 

vice of small volume similar to a current fact that the MF trapped miner device 

transformer. The loop could be a coupler could be used in nonemergency situations 

that was unwound. In either case, the as a page receiver and thereby be a cost 

antenna is small. If nearby wiring does effective addition to a general mine com- 

not exist, the loop could be deployed in munication system. Table 3-3 lists MF 

hope of coupling to distant wiring. When communication system specifications, 
so deployed, the transmitter sends out MF 

TABLE 3-3. - MF communication system specifications 

Emissions, narrowband FM: 

Occupied bandwidth kHz . . 10 

Rf frequency kHz.. 60-1,000 

Peak deviation kHz.. ±2.5 

Modulated frequency Hz.. 200-2,500 

Receiver, superheterodyne: 

Sensitivity 1.0 uV (12-db sinad) 

Selectivity 8-pole crystal filter 

IF 3-dB bandwidth (minimum) kHz.. 12 

IF 70-dB bandwidth (maximum) kHz.. 22 

RF bandwidth kHz.. 60-1,000 

Squelch Noise operated and tone 

Transmitter, push-pull, class B: 
Output power, W: 

Vest 4.0 

Vehicular 20.0 

Antenna magnetic moment (ATm^): 

Vest 2.1 

Vehicular 6.3 

RF line coupler, transfer impedance (Z-p): 
1-in coupler, ohms: 

350 kHz 10.0 

520 kHz 11.2 

820 kHz 17.8 

4-in coupler, ohms: 

520 kHz 10.6 



79 



3.7.4d Performance Data 

In order to evaluate the potential 
of MF signals as a means to locate and 
communicate with trapped miners, and to 
provide communications for the actual 
rescue team operation, a test was con- 
ducted at the York Canyon Mine near 
Raton, N. Mex. , in June 1982. This mine 
is a coal mine located in the York seam 
of the Raton Basin. The terrain is hilly 
such that the mine overburden varies from 
about 200 to 800 feet. 

The mine has four main drift entries 
that are about 7,000 feet long. Off 
these entries , submains were driven and 
longwall mining occurs. A borehole is 
located at about the 7,000-foot mark. 
This borehole contains a twisted pair ca- 
ble that is associated with the fire mon- 
itoring system on the longwall panels. 

This is an ac mine that transports 
the coal by belt. Rubber-tired vehicles 
provide transportation for personnel and 
supplies. The distance from the portal, 
down the main entries to the longwall 
faces, can be nearly 15,000 feet). 

At the mine portal, a MF signal cou- 
pler was attached to the mine telephone 
lines. This coupler was controlled by a 
standard MF base station. A second cou- 
pler and base station were placed at the 
top of the borehole. The coupler was 
clamped around the cable that went down 
the borehole. 

Two personnel entered the mine and, 
by vehicle, traveled down the main en- 
tries to the vicinity of the borehole 
(7,000 feet). These personnel were 
equipped with MF vest transceivers that 
had a magnetic moment of 2.1 ATm^ and a 
sensitivity of 1 V at 520 kHz. The in- 
tent of the test was to ascertain whether 
or not these personnel could communicate 
with the base at the portal, or the base 
at the top of the borehole. If so, it 
would demonstrate that MF-equipped rescue 
teams could communicate with the outside 
command center without deploying their 
own communications line, or relying on 
the integrity of the mine phone line that 
may, or may not, be intact. In addition. 



it would demonstrate that if a trapped 
miner was equipped with a MF transceiver 
of similar specifications, he or she 
could directly communicate with rescue 
teams in the mine, or search crews on the 
surface who were monitoring any conduc- 
tors egressing the mine. 

The result of the test showed that 
communications were possible from almost 
anywhere in the haulage and belt entries 
to either base station. It was even pos- 
sible for the base at the portal, on the 
telephone line, to communicate with the 
base atop the borehole, on the fire moni- 
tor line, even though there was no physi- 
cal connection between the two. Whenever 
a vest was within a few feet of mine con- 
ductors , there was an obvious improvement 
in clarity and signal strength. 

Although this test was preliminary, 
it clearly highlights the potential of 
using MF coimnunications for search and 
rescue operations. Much more work is 
necessary to measure range from mine wir- 
ing whenever the mine is not operating as 
would be the case during search and res- 
cue operations. An operational mine pro- 
duces considerable levels of acoustic and 
EM noise which reduces MF system range. 

3.7.5 Emergency Warning Systems 

Many types of emergency warning sys- 
tems are available for alerting under- 
ground personnel. One example is the 
stench warning system, which introduces a 
distinctive odor into the airstream. 
Visual signals or radio paging could also 
be used to alert underground personnel. 
A preferred warning system would operate 
over existing wiring, such as the twisted 
pair of a pager phone system, and broad- 
cast an audio warning that can be heard 
throughout the active areas of the under- 
ground complex. Before deciding on an 
alarm system, factors that affect the 
range over which an audio alarm can be 
heard should be considered. The most im- 
portant factors are the noise background 
found in mines, the attenuation that the 
mine environment imposes on the alarm 
signal, and the attention-getting quality 
of different alarms. 



80 



The intensity of a sound is the en- 
ergy in the sound wave. It is customary 
to express intensities or pressure levels 
in decibels. The term "loudness" refers 
to the response of the human ear to 
sound. Experiments have established that 
the loudness of a tone is a function of 
both frequency and intensity, with the 
ear most sensitive to frequencies in the 
region of 1 to 2 kHz. In other words, 
for tones with the same intensity, tones 
in the l,000-to-2,000-Hz region appear 
louder than those above or below this 
region. 

Figure 3-58 shows the noise level 
for a typical continuous miner, with 
noise samples taken at the operator's po- 
sition and with the conveyor running. To 
estimate the masking effects of these 
samples, we must first transform the 
curves so that they refer to sound levels 
on a per cycle basis. This has been done 
in figure 3-59. The center curve, la- 
beled "Mask noise source," plots the av- 
erage of figure 3-58 in terms of the 
sound level per cycle of bandwidth. The 
upper curve, labeled "Detection threshold 
at noise source," shows the estimated 
threshold level as a function of tone 
frequency. The curve shows that tones 





200 315 
FREQUENCIES 



FIGURE 3-58. - Cutting with conveyor on 
(operator position). 



FIGURE 3-59. = Detection thresholds. 

between 250 and 1,500 Hz require a level 
in excess of 80 dB to be just detectable. 
If we allow an additional 10 dB to insure 
detectability , alarm tones would have to 
have a sound level of at least 90 dB at 
the operator's position. 

If we move 15 feet away from the op- 
erator's position (the bottom curve in 
figure 3-59) , these sound levels are re- 
duced considerably. This curve shows 
that at 800 Hz a level of 60 dB is re- 
quired, and thereafter the required level 
decreases until at 6,000 Hz it is about 
40 dB. 

As mentioned earlier, the ear is 
most sensitive in the region of 1,000 to 
2,000 Hz and decreases at higher frequen- 
cies. Figure 3-59, however, shows that 
the higher the frequency of a tone (up to 
8,000 Hz), the more detectable it is. 
The spectrum of the masking noise is the 
cause of this apparent contradiction. 
The background noise is high at the 
frequencies where the ear is sensitive 
and decreases with frequency. 

In addition to overcoming background 
noise, planners must compensate for 
attenuation of the warning tone. Experi- 
mental and theoretical investigations 
are in close agreement on the attenua- 
tion of sound in room-and-pillar mines. 
Figure 3-60 shows a plan of one experi- 
ment on attenuation of sound. For this 
experiment, a 100-dBA source was mounted 
at the position shown in the figure, and 



81 



DATA GATHERING 
POSITIONS 



® 



^^ B^ ra F: 




/ 



® 



<^ 



100 

_l_ 



150 
_1_ 



SCALE (FEET) 

FIGURE 3-60. - Planof mine in whichexperiment 
was conducted. (Coal seam height, 76 inches.) 



the sound levels at the points labeled 1 
through 4 were recorded. Figure 3-61 is 
typical of the data obtained. It plots 
attenuation as a function of frequency at 
the four points. 

In practice, an audio warning source 
must be some distance from the personnel 
it is intended to alert, and it is desir- 
able that the warning be detectable above 
the background noise from as far away 
as possible. The greater the distance 
the sound must propagate, the louder the 
source must be; hence, the greater the 
hazard that the source will damage the 
hearing of someone who is inadvertently 
close to it when it is actuated. In an 
actual emergency, the risk of subject- 
ing a miner to intense sound may be 




FIGURE 3-61, - Attenuation as a function of 
frequency. 

considered justified; but to insure reli- 
ability, warning systems must be routine- 
ly tested, preferably in an operationally 
useful way, such as signaling the end of 
a shift. (Fire stations routinely test 
their sirens by sounding off at noon or 
some other prearranged time.) In addi- 
tion, any system is subject to false 
alarms and/or pranks. Considering these 
factors, the presence of a really intense 
noise source might be regarded as an un- 
warrantable menace. 

Table 3-4 combines the effects of 
background noise level and attenuation of 
the alarm tone to show the sound level 
required at the source for the warning to 
be just detectable by the operator of a 
continuous miner. The significance of 
these numbers is best explained by taking 
a particular example. The entry for 
1,000 Hz under 210 feet is 100. This 
means that at 1,000 Hz the source level 
required to just alert the operator of a 
continuous miner who has "normal hearing" 
is 100 dB when the source is 210 feet 
away from him. 



TABLE 3-4. - Sound level required at source for warning 
to be just detectable at operator's position on a 
continuous miner, dB 



Frequency (Hz) 


Distance from source 




70 ft 


140 ft 


210 ft 


280 ft 


250 


93 


96 


100 


101 


500 


95 


99 


103 


105 


1,000 


94 


96 


100 


103 


2,000 


91 


95 


99 


103 


4,000 


88 


91 


102 


107 



82 



There are systems commercially 
available that can satisfy the require- 
ments of audio warning systems using ex- 
isting mine wiring. These systems use 
the mine paging telephone network as the 
emergency alarm system. This approach 
requires the addition of an alarm signal 
generator compatible with the pager phone 
operation. The paging telephone and ex- 
ternal remote speakers act as the alarm 
sounding units. The alarm signal can be 
transmitted using a standard mine paging 
telephone and an acoustically coupled 
alarm signal generator or by using a ded- 
icated on-line alarm signal generator, as 
shown in figure 3-62. Alarm signals are 
fed onto the pager phone line in one of 
two ways. 

The first way uses the small porta- 
ble alarm signal generator shown at the 
top of figure 3-62. When operated, this 
unit emits an audio alarm via a small 
speaker. The speaker is equipped with a 
suitably sized rubber gasket that enables 
the sound to be efficiently coupled into 
the microphone of any standard pager 
phone. Units of this type are commonly 
used in conventional telephone applica- 
tions to remotely control such items as 
telephone answering machines and WATS 
line access. It can be seen that sound- 
ing an alarm in this way is a little 
awkward since three buttons must be 
pushed simultaneously, but this provides 
a safeguard against an accidental alarm. 
In addition, the portable units need only 
be entrusted to responsible individuals, 
which is a safeguard against pranksters. 



ALTERNATIVE 
^ METHODS OF 
SOUNDING 
THE ALARM 




REMOTE SPEAKER 



ALTERNATIVE ALARMS 



FIGURE 3-62. - Useof theexisting pagertele- 
phone network as an emergency alarm system. 



The second way of sounding an alarm 
on the system is to use the on-line alarm 
signal generator shown in figure 3-62. 
When the button on this unit is pressed, 
it places the correct dc signals on 
the line to actuate the pager phones 
and electronically transmits the alarm 
signal. 



BIBLIOGRAPHY 



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Mine. BuMines OFR 37-75, June 1974, 
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2. Bensema, W. D. , M. Kanda, and 
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3. Bergeron, 
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A. A. Grace Iron 
Collins Commercial 



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4. Bradburn, R. A. Communications 
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IC 8744, 1977, pp. 3-11. 



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8. Chufo, R. L. Leaky-Feeder 
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9. . Trackless-Trolleyless 

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244 900. 



12. . Radio Paging. Paper in 

Underground Mine Communications (in Four 

Parts). 2. Paging Systems. BuMines 
IC 8743, 1977, pp. 29-33. 



20. Long, R. G. , J. D. Foulkes, P. M. 
Kay, and S. J. Lipoff. Systems Study of 
Metal and Nonmetal Mine Communications. 
BuMines OFR 32-82, August 1979, 231 pp. 



13. 



Remote Control of Circuit 



Breakers. Paper in Underground Mine Com- 
munications (in Four Parts). 3. Haulage 
Systems. BuMines IC 8744, 1977, 
pp. 24-27. 



21. Murphy, J. N. , and H. E. 
Parkinson. Underground Mine Communica- 
tions. Proc. IEEE, V. 66, No. 1, January 
1978; p. 26. 



84 



22. Parkinson, H. E. , and J. D. 
Foulkes. Conventional Telephone Equip- 
ment. Paper in Underground Mine Communi- 
cations (in Four Parts). 1. Mine Tele- 
phone Systems. BuMines IC 8742, 1977, 
pp. 3-17. 

23. Sacks, H. K. Refuge Shelter Com- 
munication System. Paper in Underground 
Mine Communications (in Four Parts). 
4. Section-to-Place Communications. Bu- 
Mines IC 8745, 1977, pp. 73-76. 



24. 



Trapped-Miner 



and Communication Systems. 



Location 
Paper in 



Underground Mine Communications (in Four 
Parts). 4. Section-to-Place Communica- 
tions. BuMines IC 8745, 1977, pp. 31-43. 

25. Sacks, H. K. , and R. L. Chufo. 
Hoist Radio Communications. Paper in 
Underground Mine Communications (in Four 
Parts). 3. Haulage Systems. BuMines 
IC 8744, 1977, pp. 28-41. 

26. Spencer, R. H. , P. O'Brien, and 
D. Jeffreys. Guidelines for Trolley Car- 
rier Phone Systems. BuMines OFR 150-77, 
March 1977, 170 pp.; NTIS PB 273 479. 



85 



CHAPTER 4. —COMPUTERIZED MINE MONITORING'' 



4.1 Introduction 

Monitoring systems can have numerous 
uses in the mine. They can aid in the 
efficient management of the mine by pro- 
viding environmental trend data, pro- 
duction and maintenance control, and 
communications. In some cases, they can 
provide justification to petition the 
Mine Safety and Health Administration 
(MSHA) for a variance of one of the man- 
datory safety standards. They may also 
increase the gross revenues of the mine 
by increasing the amount of coal produced 
or increase profits by reducing the cost 
of producing that coal. 

No single system will satisfy the 
requirement of all mines. Some may re- 
quire simple hard-wired status-reporting 
systems; others, multipurpose computer- 
based systems that collect, analyze, and 
store data and perhaps control some mine 
functions. Even though systems vary in 
complexity, they are all composed of 
three functional components. The first 
component is sensors that measure the en- 
vironmental or production parameters and 
produce an electrical signal that is fed 
into the telemetry. The second is telem- 
etry devices that receive the signal from 
the sensors and transmit it in either an- 
alog or digital format to the third com- 
ponent, analysis and display equipment. 
This equipment receives the transmitted 
signal and either stores it for later 
analysis or displays it. The analysis- 
display equipment ranges from simple 
strip chart recorders with preset alarms 
to computers, cathode-ray tubes (CRT's), 
and line printers that can also provide 
production reports. 

4.2 Uses of a Mine Monitoring System 

A list of potential uses for 
mine monitoring systems, including both 



production-related functions and those 
related to health and safety, was used to 
develop a questionnaire. It was present- 
ed to representatives of the mining com- 
munity to determine their current moni- 
toring priorities. 

The responses indicated that the in- 
dustry's priorities fall into the follow- 
ing two categories: 

First priority — 

Production and haulage 

Maintenance 
Second priority — 

Ventilation 

Communication 

Fire monitoring 

Personnel 

The survey shows that production- 
oriented systems were the most appealing 
to the questionnaire respondents. Since 
even small improvements in production ef- 
ficiency and maintenance can have a large 
financial impact, the desirability of 
monitoring systems that focus on these 
areas is understandable. 

The results are summarized in table 
4-1. The function that scored 100 was 
viewed as the most beneficial monitoring 
function. 



^ From Guidelines for Environmental 
Monitoring in Underground Coal Mines. 
Phase I Report. BuMines OFR 180-82, 
1982, 177 pp.; NTIS PB 83-147777. 



86 



TABLE 4-1. - Survey 

1. Output by section 

2. Belt monitoring and control.... 

3. Scheduled routine maintenance.. 

4. Equipment repair history 

5 . Spare parts inventory 

6. Assist in diagnosis of failure. 

7. Power: Fault location 

8. Face equipment operating time.. 

9. Personnel: Shift organization.. 

10. Ground fault detection 

11. Trailing cable failure 

12. Power center monitoring 

13. Monitor car haulage system 

14. Personnel: Locate skills; 
assist in locating people with 
needed or critical skills 

15. Ventilation: Eliminate inspec- 
tions; eliminate or reduce 
frequency of some periodic 
inspections 

16. Communications: Reliability.... 

17. Communications: Intelligibility 

18. Paging 

19. Plan new ventilation; help in 
planning ventilation, including 
new ventilation shafts 

20. Inspection scheduling; alert 
foremen or others to scheduled 
or predictable inspection or 
repair 



results, weighted rank score 



100 
99 
99 
97 
95 
94 
73 
68 
66 
63 
62 
62 
59 

58 

50 
49 

45 
45 

45 
43 



21. Fire: Beltways; a system to 
detect and warn of incipient 
fires in the beltway due to hot 
rollers or other problems 



22. 



Beltway for intake air; the use 
of improved fire detectors and 
monitoring system so as to 
qualify for a variance and ena- 
ble use of the beltway for in- 
take air 



23. 

24. 

25. 

26. 

27. 
28. 

29. 

30. 

31. 

32. 
33. 



Communication: 
station 



Station-to- 



Emergency signaling; direct 
people during any emergency by 
signaling 

Detect leaky stoppings; venti- 
lation monitor to detect open 
doors, blockages in air course, 
and leaky stoppings 

Personnel: Emergency; assist 
in locating and aiding people 
during an emergency 



Fire: Haulage; detect incipi- 
ent fires in trolleyways 



Ventilation: Control regula- 
tors; monitor ventilation, and 
adjust regulators to improve 
flow distribution 



Roof fall prediction; automat- 
ically plot falls and/or micro- 
seismic activity to predict 
roof fall 

Cage; monitor the operation of 
the cage to predict failures or 
minimize delays 

Fire: Gob; monitor gob areas 
for fire 



42 



Inventory expendables, 
Rock bursts 



41 
40 

40 

37 

30 
29 

25 

20 

18 

18 
10 
10 



87 



Responders were also asked to indi- 
cate the relative importance of other 
cost and technological factors that may 
affect user acceptance. Results, on a 
100-point scale, were 

1. Reliablity of monitoring equip- 
ment in mine environment 100 

2. Maintenance cost 90 

3. Initial cost 88 

4. Skills required to maintain 
equipment 75 

5. New technology to mining 62 

Reliability of the equipment was the 
most frequently cited "very important" 
factor. 

4.3 Petitions for Modification 

Mine monitoring systems can be used 
to provide a cheaper and safer alterna- 
tive to satisfying the mandatory safety 
standards set forth in 30 CFR 75, provid- 
ed that the alternate method (in this 
case, the monitoring system) guarantees 
no less than the same measure of pro- 
tection afforded by the standard (30 CFR 
44) . The extent to which the industry 
currently takes advantage of these us- 
ages can be determined by reviewing the 
Petitions for Modification of Manda- 
tory Safety Standards. 2 Since ventila- 
tion regulations appear to be the most 
likely candidates for modification peti- 
tions, petitions were reviewed under sub- 
part D, "Ventilation," in the following 
sections: 

75.305 Weekly examinations for 
hazardous conditions. 

^Sources include the Federal Register, 
the Bureau of National Affairs, Inc., 
"Mine Safety and Health Reporter," and 
the McGraw-Hill 1979 "Guide to Modifica- 
tion of Safety Standards in Coal Mines." 



75.306 Weekly ventilation 
examinations. 

75.307 Methane examinations. 

75.310 Methane in virgin territory. 

75.326 Aircourses and belt haulage 
entries. 

This review identified a number of 
cases where continuous monitoring was 
used in a petition for a variance and a 
number of others that could have used 
continuous monitoring. Included in the 
review were petitions that were granted 
and petitions that were filed, but not 
acted upon as of the writing of this re- 
port. General comments on the petitions 
follow. 

75.305 Weekly Examinations for Haz- 
ardous Conditions. - This section re- 
quires weekly inspection of at least one 
entry of each intake and return air- 
course, in its entirety, for both methane 
and for compliance with the mandatory 
health and safety standards. Typical 
petitions state that because of poor roof 
conditions it is not possible to travel 
the aircourses in their entirety, and 
offer checkpoint measurements as an al- 
ternative. Continuous methane (75.305) 
measurements could be made with a moni- 
toring system at these checkpoints. Re- 
quired airflow measurements (75.306) 
could also be made with the same system. 

Only 1 of the 62 petitions that were 
granted offered continuous monitoring. 
An additional 20 petitions were filed, 
but there was no record of any final de- 
cisions. One of these petitions did pro- 
pose to install two methane monitors at 
specified points. 

75.307 Methane Examinations. - This 
section requires tests for methane at 
each working place immediately prior 
to energizing electrically operated 
equipment. 



88 



One petition was noted in which 
methane monitoring devices were installed 
on permissible electric water pumps in 
the face area to eliminate the meth- 
ane examinations by a qualified per- 
son required prior to energizing the 
pumps. 

75.310 Methane in Virgin Terri- 
tory. - This section requires that all 
electric power be cut off and men 
withdrawn when air returning from vir- 
gin mining areas contains 2% or more 
methane. 

Three petitions for modification 
were granted under the stipulation that 
continuous automatic methane monitors 
were used in the return as an alternative 
to measurements made by certified mine 
personnel. 

75.326 Aircourses and Belt Haulage 
Entries. - This section requires that en- 
tries used as intake and return air- 
courses be separated from belt haulage 
entries. 

Fourteen petitions that were grant- 
ed and ten that were filed but not act- 
ed upon were reviewed. Of these, seven 
petitions were granted on the basis 
of continuous monitoring systems , and 
seven of the filed petitions proposed 
continuous monitoring of carbon mon- 
oxide. A review of MSHA tests that 
demonstrate the "equivalency" of car- 
bon monoxide sensors and the customary 
point-type heat sensors is presented in 
reference 15. 

In summary, at least 11 continuous 
monitoring systems have been installed in 
U.S. underground coal mines for purposes 
of obtaining a variance from the manda- 
tory health and safety standards. Eight 
additional petitions for modification 
mention such systems. 



4.4 Technical Factors 

The key technical issues are whether 
the sensors can actually provide the 
needed input information, the ability of 
the processing system to interpret cor- 
rectly the telemetered information, and, 
finally, overall system reliability. 

4.4.1 Sensors 

Sensors are the critical element in 
mine monitoring systems since they pro- 
vide the input data. If the input data 
are not correct or are not representative 
of the required measurement, the entire 
monitoring process is meaningless, i.e., 
"garbage in, garbage out." One problem 
with sensors is that their output repre- 
sents the response of the sensor to a 
number of parameters in addition to the 
one that is to be measured. Typical ex- 
amples are the response to changes in 
temperature and the poisoning of environ- 
mental sensors by other gases in the 
mine. 

The critical problem relates to the 
ability of the sensor actually to measure 
the parameter of interest. In particu- 
lar, ventilation monitoring systems use 
point air velocity measurements to repre- 
sent the total airflow at a cross section 
in the mine. The total airflow is deter- 
mined either from an empirically derived 
factor and the point measurement or from 
actual calibration of the cross section. 
The problem is further complicated be- 
cause the only safe location for the 
sensor is on the rib or roof in the low- 
flow boundary layer. It is possible to 
have large changes in the overall airflow 
with little or no change in the veloci- 
ties in the boundary layer and conse- 
quently in the sensor output. The reader 
is referred to reference 13 for guide- 
lines for avoidance of these problems in 
airflow measurement. 



89 



4.4.2 Telemetry 



4.4.3 Reliability 



The telemetry system obtains the 
data from the sensor, converts them to a 
standard format, sends them to another 
unit that receives them, checks their 
authenticity, and then refers them to the 
analysis-display device. The principal 
problem in this area is data security, 
i.e., the error rate for information 
transmission. The problem is not so much 
that an error is transmitted but that an 
error in transmission goes undetected be- 
cause of the noise on the transmission 
line. The sensitivity to erroneous data 
transmission depends upon factors such as 
the cable used, the local noise field, 
length of cable run, and data formatting. 

Techniques for detecting erroneous 
data transmissions have been devised 
principally by computer manufacturers. 
Notable among these are IBM's synchronous 
data link control (SDLC) and Digital 
Equipment Corp.'s digital data communica- 
tions message protocol (DDCMP). 

Bureau research (iL~A^»^ indicates 
that the maximum transmission distance 
for one undetected random error per year 
varies between 1.3 and 6.8 miles in an 
average noise field, and between 0.1 and 
0.6 mile in an estimated maximum noise 
field. Since cable runs are frequent- 
ly several miles, occasional undetected 
transmission errors can be expected. For 
typical monitoring applications with fre- 
quent data refresh, this should not be 
a factor that causes worry; however, in 
the case of control systems or the least 
favorable monitoring circumstances, er- 
ror rates can be unacceptably high, and 
corrective measures such as more secure 
transmission systems and improved error 
detection protocols are necessary. 



■^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this chapter. 



The final area of concern is system 
reliability. The questionnaire identi- 
fied reliability as the prime concern. 
The Bureau is currently sponsoring re- 
search that provides a methodology for 
determining the reliability of systems 
(12, 17 , 21). This methodology has been 
used to evaluate expected failure rates 
of current mine monitoring systems. 

Reliability in monitoring systems 
takes a number of forms. The first is 
mechanical reliability of the components. 
The underground mining environment is no- 
toriously hard on equipment because of 
water, dust, potential damage due to mov- 
ing equipment, and rough handling. 
Therefore, the enclosures for remote sta- 
tions should be rugged enough to with- 
stand the day-to-day rigors typically 
encountered in underground service. The 
enclosures should have tight and durable 
seals if the internal components are 
sensitive to moisture or dust. All ex- 
terior switches and buttons should also 
be sealed or be durable enough to with- 
stand constant use in the presence of 
dirt and moisture. Cables should be dur- 
able enough to withstand occasional rough 
treatment. 

The second aspect of reliability is 
electrical power reliability. Since pow- 
er outages are all too common in under- 
ground mining, some type of backup power 
or uninterruptable power supply should be 
provided for this system. Such a power 
supply is particularly important for mon- 
itoring systems that provide essential 
health and safety information such as 
main fan operation, fire detection, and 
methane content. Obtaining these data is 
important during the common day-to-day, 
short-term power outages, but it is just 
as important to have such information 
during emergency situtions such as roof 
falls, fires, or explosions. It is also 
required for system approval. 



90 



4.5 Commercially Available Mine 
Monitoring Equipment 

4.5.1 Introduction 

The mine monitoring systems dis- 
cussed in this report are electromechan- 
ical systems that remotely sense various 
environmental and operational parameters 
and transmit the data to a central loca- 
tion where the data are analyzed and/or 
displayed. On the basis of this defini- 
tion, it is reasonable to discuss the 
system in terms of three basic functions: 
sensing, data transmission (or telemetry) 
and data analysis and display. In the 
case of monitoring and control systems, 
such as systems that automatically and 
remotely deenergize face equipment when 
the methane content at a specified lo- 
cation reaches a predetermined level, 
the control operation presents a fourth 
function. 

Sensing can be divided into two gen- 
eral categories: environmental and oper- 
ational or production sensing. The first 
category of sensors is designed to mea- 
sure various aspects of a mine's environ- 
ment to assist in maintaining a safe en- 
vironment for underground personnel. The 
parameters that are ordinarily of concern 
are gas (i.e., carbon monoxide, methane, 
oxygen, etc.) content, air velocity, air 
temperature, differential pressure, and 
humidity. Typically, the data are used 
to detect and locate potentially hazard- 
ous conditions (i.e., fires, gas bursts, 
etc. ) so that the appropriate measures 
can be taken. Production sensors are 
used to monitor the operating status of 
various pieces of underground equipment 
to detect production bottlenecks, equip- 
ment malfunctions, maintenance require- 
ments, etc. Examples of production pa- 
rameters that are typically of interest 
are belt output, face equipment opera- 
tion, belt slippage, blockages, and bear- 
ing temperatures or vibration. 

Telemetry is the process of trans- 
mitting the data output of the sensors to 



the control center that is usually lo- 
cated aboveground. The output of the 
sensors can be either a simple status in- 
dication, sometimes called a binary, con- 
tact closure, or status output (such as 
high-low, open-closed) or it can be a 
continuously variable function of time 
(such as air velocity, methane concentra- 
tion, etc.). While the continuously var- 
iable data can provide significantly more 
information than the simple status data, 
how much more depends on the accuracy of 
the measurement. 

As a practical matter, it is gener- 
ally not feasible to run a separate con- 
ductor or conductor pair to each sensor. 
Therefore, telemetry systems typically 
incorporate several remote stations or 
"outstations , " each of which accepts and 
encodes the output of a number of sensors 
and transmits the encoded data along a 
common cable to the control center. The 
two most common encoding techniques are 
(1) frequency domain multiplexing and (2) 
time domain multiplexing. Frequency do- 
main multiplexing has the advantage that 
data from all monitoring points are re- 
ceived at all times, although the number 
of monitoring points is limited by the 
overall bandwidth of the system. Time 
multiplexing can be expanded, at least in 
principle, to accommodate as many moni- 
toring points as desired. However, each 
point is sampled only intermittently 
(i.e. , the receiver obtains data from 
only one monitoring point at a time) 
since the system interrogates the moni- 
toring points sequentially. The cycle 
time, or time between successive sam- 
plings of the same point, is the time the 
system requires to interrogate all of the 
monitoring points. 

Time multiplexed systems, the more 
common of the two, often transmit data in 
the digital format. That is, a series of 
high-low state indications is transmitted 
to indicate the status of the monitor 
point. A common technique to accomplish 
this transmission is to use frequency 
shift key (FSK) encoding. This encoding 



91 



process uses two different frequencies 
(for example, 3,000 and 2,000 Hz) to rep- 
resent the high and low states, rather 
than high and low level signals of the 
same frequency. The FSK encoded data are 
less affected by noise on the transmis- 
sion line than data transmitted in simple 
high-low digital format. In addition, 
current signal detection techniques make 
it very easy to detect single frequency 
signals in the presence of noise. 

The third basic function of a moni- 
toring system is the analysis and display 
of the measured data. These operations 
are normally accomplished in an above- 
ground control center. Most of the sys- 
tems have the ability to trigger audio- 
visual alarms if a sensor detects that 
its predetermined threshold (such as 1% 
methane in a return airway) has been ex- 
ceeded. Most of the systems can also 
provide hard copy documentation of the 
alarms and display the actual values de- 
tected by the sensors, either on meters 
or CRT's. The computer-based systems 
have the added capability of data storage 
for trend analysis, record keeping, and 
reporting. 

In the following discussion on com- 
mercially available equipment, a distinc- 
tion is made between the system suppliers 
and the sensor suppliers. The distinc- 
tion is made since in many cases the 
system supplier expects the mine to 
select not only the parameters to be 
monitored but also the sensors to be 
used. The system supplier then config- 
ures a monitoring system using both in- 
house hardware and hardware from outside 
suppliers to provide the mine with the 
desired information. Ordinarily, the da- 
ta telemetry-analysis and display equip- 
ment is the supplier's own brand, while 
the sensors are obtained from outside 
companies. In most cases, the supplier 
will assume "full system responsibility." 
That is, it will not only provide the 
telemetry-analysis and display equipment, 
but will also ensure proper interfaces 
for any sensors selected by the mine, 
provide software to process the data. 



and assist the mine during installation, 
testing, and operator training. The 
costs for these services, however, may 
sometimes be broken out separately from 
the hardware costs. For mines that pre- 
fer to use in-house personnel for these 
tasks, systems suppliers that restrict 
themselves to providing the hardware 
alone may be worthwhile. 

4.5.2 Telemetry-Analysis and Display 
Systems 

Table 4-2 summarizes the major mine 
monitoring systems currently available in 
the United States. Three of the systems, 
Davis, Hawker Siddeley, and Transmltton, 
were originally based on the MINOS system 
developed by the National Coal Board in 
Great Britain. Systems offered for sale 
in the United States may, however, differ 
from the original MINOS system. Most of 
the systems have the capacity for accept- 
ing input from a wide variety of environ- 
mental and production sensors. An indi- 
cation of the extensive use of monitoring 
systems abroad can be obtained by compar- 
ing the U.S. -foreign installations for 
the two British systems. While these 
systems are used extensively abroad, they 
are just beginning to be accepted in the 
United States. 

As discussed previously, most of the 
systems use an FSK format for data trans- 
mission. The exceptions are Conspec, 
Mundix, and Transmitton, which use a 
direct binary transmission. 

While the range in the number of 
monitoring points and cable length is 
substantial, most systems should provide 
sufficient capacity for typical usage. 

In terms of system costs, one 
manufacturer uses a "rule-of-thumb" of 
$50,000 for the central station and 
$20,000 per mine section. 

The final category of table 4-2 in- 
dicates which suppliers usually assume 
overall system responsibility. 



92 



TABLE 4-2. - Mine monitoring systems currently available in the United States 





Aquatrol 


Conspec 


Davis 


Giangarlo 


Hawker 
Siddeley 


Kidde 


Current installations: 
Coal mining. ............ 


1 

1 

3,000 

FSK 

NA 

2 

'1 

110 
NAp 

40-75 
5-10 

Yes 


4 

12 

Several 

DB 
768 

4+S 

10.8 

110 
12 

25 
0.25 

Yes 


1 



FSK 
'5,080 

2+S 

4+S 

20 

110 
NAp 

100 
25 

Yes 


14 

8 

FSK 
26,400 

2+S 

20 

110 
12 

15-23 
33-7 

Yes 


>100 
NA 
NA 

NA 
'6,944 

7+S 

8 

110 
NAp 

NA 
NA 

Yes 


4 


Metal-nonmetal mining... 
Other industries 

Specifications: 

Data transmission 

Maximum number of moni- 
toring points. 

Cable (number of con- 
ductors). 

Maximum cable miles., 
length. 

Power requirements, V: 
Ac 



100 

FSK 
1,024 

1+S 

Coax. 

10 

110 


Dc 


NAp 
75-100 


Cost, thousand dollars: 
Central station. ........ 


Outstation. ............. 


3-4 


Overall system 
responsibility. 


Yes 




MSA 


Mundix 


Outokompu 


R.F.L. 


Sangamo 
Weston 


Trans- 
mitton 


Current installations: 
Coal mining. ............ 


1 



FSK 
72,000 

2+S 

8 

110 
12 

25 

1 

Not 
normally. 




1 
1 

DB 
24,096 

4+S 

(HF) 

128 

110 
24 

80 
3 

Yes 




1 
>100 

FSK 
Unlimited 

2+S 

>6 

110 
24 

5-100 
3.3 

Yes 


1 



100 

FSK 
4128 

2+S 

>10 

NAp 
12 

1.3-1.5 
0.7 

Not 
normally. 




1 

>100 

FSK 
8,192 

2+S 

4+S 

'2 

110 
12 

15-100 
5-15 

Not 
normally. 


200 


Metal-nonmetal mining. . . 
Other industries 

Specifications: 

Data transmission 

Maximum number of moni- 
toring points. 

Cable (number of con- 
ductors) . 

Maximum cable miles., 
length. 

Ac 





DB 
7,392 

4+S 

10 

110 


Dc 


NAp 

50-60 


Cost, thousand dollars: 
Central station 


Outstation. ............. 


4-8 


Overall system 
responsibility. 


Yes 



DB Direct binary. 

FSK Frequency shift key, 

'Extendable. 

2Digital inputs. 



NA Not available. 
NAP Not applicable. 
^Includes CO monitor. 
''# per station. 



S Shield. 



93 



4.5.2a Systems Suppliers 

The system is typically purchased 
from a vendor who will supply the 
telemetry-analysis and display equipment. 
Since this vendor will usually ask the 
mine operator to specify the sensors used 
in the system, sensor operating princi- 
ples and available sensors are discussed 
separately. 

All of the systems suppliers, with 
the exception of R.F.L. , provide com- 
puter-based systems with printers, CRT's, 
software packages and auxiliary power 
in case of main mine power failure. The 
listing is of necessity incomplete, it 
does not represent endorsement by the 
Bureau of Mines, nor is responsibility 
assumed for any errors that may have 
occurred in system performance descrip- 
tions. Much of the material was ab- 
stracted from telephone conversations 
with and brochures received from the 
designer-manufacturers . 

Aquatrol Corp. 
2258 Terminal Road 
St. Paul, MN 55113 
(612) 636-3950 

Aquatrol Corp. markets an Intel 
8085-based monitoring and control system 
that is sold primarily to water treatment 
facilities. It has full system capabil- 
ity including color CRT, printer, and 
262,000 random-access memory (RAM) floppy 
disk storage. The system will accommo- 
date up to 98 outstations on the master 
trunk, with up to 18 data channels per 
outstation. Each channel can be analog, 
resolved to 12 bits, or the 12-bit digi- 
tal word can be treated as individual 
binary level inputs. 

Similarly, each outstation can out- 
put up to 6 analog channels or 12 binary 
drive levels for each analog channel less 
than 6. 

The cable is two-conductor voice 
grade, operable to 5,000 ft or, with an 
extender, further. Power is 110-volt ac 
or 12-volt dc for both the central or the 
remotes. 



Telemetry is at 
RS232C format. 



300 baud using 



A training program about a week long 
is offered at their plant. They will 
assume system responsibility and quote a 
maintenance contract if desired. 

Conspec Controls, Inc. 
901 Fuhrmann Blvd. 
Buffalo, NY 14203 
(716) 854-4769 

The Conspec Senturion Series 200 
monitoring system consists of a central 
processor that has a CRT display and key- 
board, two printers, one or more "data 
concentrators," which are interface de- 
vices, and a capability for up to 768 
sensing (monitoring and/or control) 
points on any of four trunks, each capa- 
ble of carrying 128 locations. At the 
sensing location, the electronics neces- 
sary to interface a sensor onto the trunk 
(including entering its address) are 
housed on a 4- by 6-inch accessor card, 
which in many cases is incorporated into 
the sensor package. 

Transmission is over four-conductor 
cable; two for power and two for signal. 
The power required is up to 30 volts dc, 
and the maximum end-to-end length is 
4,000 ft. If greater distances are re- 
quired, a trunk extender can be used; 
powered by 110 volts ac, it permits oper- 
ation to 40,000 ft. A 600-baud (bit per 
second) FSK telephone modem is also 
available. 

Data transmission is in a noise- 
resistant binary format. There are no 
remote stations, other than the accessor 
at each measurement location. Either 
binary or analog data can be transmitted. 

Display is in a 20-character alpha- 
numeric format (20 letters or numbers) so 
that interpretation is simplified. A 
complete software package is offered that 
has the capability to alarm on thresholds 
and set high-low points. Set points can 
be entered from the keyboard. There is 
central reset of remote points, a pro- 
gramed restart or load-shedding feature. 



94 



time of day or week programing, and se- 
quencing initiated by alarm or keyboard 
entry. 

Alarm states are printed and dis- 
played separately. Routine information 
such as end of shift reports may also be 
generated. 

Standard accessors available include 
interfaces for thermistors, pressure, 
RTD, power, current (4-20 mA, 0-5 volts, 
0-1 mA) , potentiometer, alarm states, 
two- or three-state load control, and 
electrical demand. 

John Davis and Sons (Derby) Ltd. 
Alfeton Road 
Derby DE2 4AB 
England 

Service Machine Co., Inc. 
Box 8177 

6072 Ohio River Road 
Huntington, WV 25702 
Attn: Mr. J. H. Nash 



A variety of system configurations 
is available. Outstations have been 
designed primarily for haulage, machin- 
ery monitoring, and communication. For 
example, the type 25200 equipment ac- 
cepts eight thermal probes and two other 
transducers, such as pressure and flow. 
Thresholds are set locally with a poten- 
tiometer. Such a unit would be appropri- 
ate as a compressor, fan, or pump moni- 
tor. There are local and remote stop 
modes. 

Similarly, the FMSl type 25000 ac- 
commodates six transducer inputs, and has 
six relay outputs for indication and/or 
control. It can perform level detection, 
temperature detection, and time delay 
functions. Applications listed include 
haulage, bunker, pump, and refrigeration 
plant monitoring. It is powered with 
110-volt main power. 

Other communication, conveyor con- 
trol, and signaling devices are also 
offered. 



Davis of Derby offers a data trans- 
mission system consisting of a surface 
master station with two video displays, a 
control keyboard, and a communications 
switchboard. Associated with this cen- 
tral station are up to 127 outstations, 
each capable of monitoring up to 40 
transducers. Interconnection is by ei- 
ther two- or four-conductor shielded ca- 
ble. Telemetry is accomplished digitally 
with FSK coding. 

Outstations can be wired to accommo- 
date prestart warning, belt slip, and 
other transducers, including temperature 
and pressure. 

Virtually all Davis of Derby under- 
ground equipment is housed in flameproof 
housings, and most circuitry is designed 
to be intrinsically safe. Certification 
and design are to international stan- 
dards, such as Cenelec Standard 50 020 
and IS1902 Class I. Circuitry is low- 
power CMOS. Standby battery power is 
available with automatic changeover. 



Giangarlo Scientific Company, Inc. 
2500 Baldwick Road 
Pittsburgh, PA 15205 
(412) 922-8850 

The Giangarlo system consists of a 
central processor with associated outsta- 
tions. The outstations are microcomputer 
controlled and are capable of accepting 
up to five input boards. Each board may 
consist of either 8 analog inputs, 16 
digital inputs, or 8 relay inputs, and 
each board has light-emitting diode (LED) 
status lamps for troubleshooting. There 
is battery backup power. Telemetry re- 
quires a three-conductor shielded cable 
with data packaged in an ASCI II serial 
FSK digital format, run at 300 to 1,200 
baud. Remote stations can alarm on their 
own, with visual and audible alarms. 

At the central computer, software 
is available that will use redundancy 
checks of data to identify faulty trans- 
mission. It can display data, set remote 
alarm thresholds, and perform control 



95 



functions. Central computer alarms are 
independent of remote alarm status. 

There is provision for interface 
from the central computer to a teletype, 
CRT display, disk memory, or another 
computer. 

Input parameters that might be 
measured and transmitted to the central 
station include carbon monoxide, carbon 
dioxide, methane, temperature, air veloc- 
ity, radon, belt slippage, belt speed, 
weight, pressure, power, etc. 

Hawker Siddeley Dynamics 

Engineering Ltd. 
Manor Road 

Hatfield, Hertfordshire ALIO 9LP 
England 
Hatfield (07072) 68234 

United Technologies Bacharach 
301 Alpha Drive 
Pittsburgh, PA 15238 
Attn: Mr. David M. Nelson 
(412) 784-2137 

Hawker Siddeley is one of the quali- 
fied manufacturers of the MINOS system. 
It offers systems for environmental moni- 
toring, conveyor and bunker control and 
monitoring, and mine cage monitoring. 
The "Dynalink" system for conveyor con- 
trol and monitoring consists of a surface 
control center with provision for local 
control and monitoring or for remote 
control from the remote station. Out- 
stations are connected to the central 
station with six-conductor cable at dis- 
tances up to several miles. Sixteen 
outstations can be carried on one cable, 
and seven cables can be accommodated 

The control station has a dual visu- 
al display. Monitored data, or change of 
state, or even mimic diagram graphics can 
be displayed. The central computer is a 
DEC PDP U/34 or CAI ALPHA LS1-2/20G. 
Microprocessors used are Intel 8080 and 
8085. There is dual control with auto- 
matic switchover for reliability. 

Each outstation can accommodate 32 
input channels, expandable in increments 



of eight. Power is 110 volt, 50 Hz, or 
any standard mine supply. 

The bunker-conveyor monitoring and 
control system can automate haulage from 
the vicinity of the face to the sur- 
face, including bunker monitoring and 
metering. 

Their mine cage monitor maintains 
the cage within speed and distance lim- 
its, comparing cage performance to a de- 
sired operation profile every 0.1 sec and 
making the appropriate corrections. 

The environmental monitor can accept 
either analog or digital inputs from 
sensors such as anemometers, methanom- 
eters, pressure sensors, and bearing tem- 
perature probes. 

Kidde Automated Systems 
(Formerly S.R. Smith Co., Inc.) 
7256 County Line Road 
Deerfield, IL 60015 
(312) 272-8012 

The S.R. Smith System uses a remote 
"data collection panel" architecture. 
Each remote station is capable of receiv- 
ing up to 16 contact closure inputs. The 
remote stations also contain relays that 
are capable of control functions such as 
starting-stopping belts. A new system 
has a capability for the transmission of 
analog levels. 

The central computer facility uses 
Digital Equipment Corp. display and anal- 
ysis equipment. A CRT displays informa- 
tion from any of 1,024 monitor points, 
connected to any of 64 remote stations 
per channel. Telecommunication is over a 
two-wire pair, or coaxial line, or 3002 
Telco for each channel. 

A complete interactive software 
package is available that displays alarms 
in a prioritized list. Data are also 
printed on a high-speed printer, as an 
aid to failure diagnosis and as a perma- 
nent record. Up to 700 characters of 
description and instruction are possible 
per alarm point. 



96 



Intended applications include belt 
monitoring (slip, alignment, power, chute 
plugging, etc.), power center monitoring 
(breaker position, voltage in range, open 
ground, etc.). environmental monitoring 
(methane within limits, air velocity 
within limits, etc.), fire monitoring, 
and fan monitoring (operating, tempera- 
ture in range, etc.). The system can be 
used with a card reader to control and 
monitor people underground or in a con- 
trolled area. 

Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 
(412) 273-5000 

Catalyst Research Corp. 
3706 Crandall Lane 
Owing Mills, MD 21117 
(301) 356-2400 

Catalyst Research Corp. has designed 
and tested a computer based supervisory 
control and data acquisition system 
(SCADA) which is also available from MSA. 
The system can accommodate up to 38 field 
data stations. Each data station is ca- 
pable of receiving eight analog inputs, 
plus any one of the following: 16 digi- 
tal inputs or 4 digital outputs. 



Mundix Control Systems, Inc. 
5495 Marion St. 
Denver, CO 80216 
(303) 296-1790 

Mundix offers a supervisory control 
and data acquisition system that can ac- 
commodate up to 128 outstations. Each 
outstation can be configured with four 
electronic interface cards (extensible 
to 16). Each card can accommodate any 
one of the following: four analog in- 
puts (12-bit resolution), eight digital 
inputs, eight digital outputs, or four 
analog outputs. 

The total system capacity is 4,096 
digital inputs or 1024 analog. Telemetry 
to the central station is accomplished 
using a 500-kHz digital phase modula- 
tion technique (Manchester coding). As 
a result, the system uses two-conductor 
shielded cable. The maximum transmission 
distance is stated to be 128 miles. 

The central station is computer- 
based, and BASIC language programable. 
There are printers and color or black and 
white CRT monitors available. Power to 
the central station is 110-volt ac, while 
remotes can be powered with either 110- 
volt ac, 24-volt dc, or 120-volt dc. 



Remote stations can be up to 18 
miles from the central station. Cabling 
is by two-conductor No. 19 gage twisted 
pair. The central station is computer 
based. It uses a Digital Equipment Corp. 
LSIll microprocessor with 64,000 bytes of 
memory. Data transfer from remotes to 
the central station uses RS232 at up to 
2,400 baud. 

Interactive, user friendly software 
is available. Alarm states are indicated 
audibly and automatically printed. Hour- 
ly and daily summaries can be generated. 



Outokumpu Engineering 
4680 Packinghouse Road 
Denver, CO 80216 
(303) 371-0540 

There are hundreds of Outokumpu mon- 
itoring systems installed in Europe, pri- 
marily in power control applications. It 
is a computer based system (DEC) with an 
interactive English control software. 
The system has a capability for up to 
30,000,000 bytes of storage on a Winches- 
ter disk and printer, CRT options. 



97 



The remote outstations are typical- 
ly configured to accept 4 analog in- 
puts, 16 digital inputs, and 16 digi- 
tal control outputs. There are two types 
of outstations, the miniremote described 
above, and larger, higher capacity units. 
Outstations can be interconnected in 
any series parallel configuration. 
Analog data is resolved to a 12-bit 
precision. 

Telemetry is accomplished using a 
serial RS232 FSK format at 300 baud (50- 
600) . Outstations can be up to 6 miles 
distant. Two-conductor twisted pair ca- 
ble is required. 

Power to the central and remote sta- 
tions is 110-volt ac, 220-volt ac, or 24- 
volt dc. 

R.F.L. Industries, Inc. 
Boonton, NJ 07005 
(201) 334-3100 

R.F.L. Industries manufactures te- 
lemetry equipment consisting of build- 
ing blocks that can be combined to 
satisfy progressively more demanding 
system requirements. The simplest sys- 
tem consists of frequency-multiplexed 
transmitter-receiver pairs that operate 
with center frequencies between 300 Hz 
and 30 kHz in spacings of 100 and 120 Hz 
or more. These devices are available in 
two configurations: an AM system, in 
which the carrier is keyed on or off with 
a 12-volt dc input status, or a system 
with a switch closure. A more secure 
frequency-shifted version uses the same 
audio band center frequencies, but 
frequency-shift codes the data in any of 
four codes including a two-frequency code 
(mark-space) and a three-frequency code 
(mark-center-space) . Both systems are 
powered by 12-volt dc. If standard CCITT 
channel spacing is used, there are 46 
carrier channels between 300 Hz and 10 
kHz, with more at higher frequencies. 



If there is a need to transmit ana- 
log data, the model 64B series converts 
input voltages in the 0.4- to 2-volt 
range, or input current 4 to 20 mA, 10 to 
50 mA, etc., into a square wave output, 
frequency coded in the 5- to 25-Hz range. 
After voltage- or current-to-frequency 
conversion, these low-frequency codes are 
transmitted by using the frequency shift 
transmitter-receiver described. 

If large numbers of status-control 
signals are to be telemetered, the mod- 
el 66A encoder-decoder will accommodate 
16 input channels. These data are 
then time-division-multiplexed (TDM) 
with a redundant transmission (normal 
and polarity-inverted, doublescan format) 
to ensure reliable transmission. There 
are two parity check bits, so that par- 
ity errors up to the third order can be 
detected. Once again the data are 
telemetered with one of the frequency- 
shift, frequency-multiplexed transmitter- 
receiver pairs. The scan time with the 
normal 60-baud system is 1.1 sec with 
doublescan and 0.6 sec with single scan. 
Input data can be binary level or switch 
closure inputs. Power is 12 volts dc. 

Analog channels can also be accommo- 
dated on the TDM system. Analog inputs 
are channeled, one at a time, to an A-D 
converter. The analog quantity is digi- 
tized and this parallel digital signal, 
together with a corresponding digital ad- 
dress, is fed to a 66A encoder. Trans- 
mission is now similar to that in the 
foregoing paragraph. At the receiving 
station, the message is decoded, and the 
digital output is channeled to the corre- 
sponding D-A converter to produce an ana- 
log output. 

Sangamo Weston, Inc. 
Industrial Products 
P.O. Box 3041 
Sarasota, FL 33578 
(813) 371-0811 



98 



Sangamo Weston manufactures three 
different monitoring and control systems. 
Their RECON I has a capability for up to 
127 remote stations, each with 12 analog 
inputs, resolved to 12-bit precision, and 
12 output levels. Channels can be ex- 
tended fourfold as an option. Operation 
is either manual or by computer control. 
Cycle time is 0.5 sec per channel. 



The RECON II has eight 
remote and can accommodate 
channels each, resolved 
precision (0.4%). It is 
an Intel 8080. The output 
terminal. Cycle time is 
channel. 



channels per 
eight analog 
to eight-bit 
controlled by 
is a printer 
0.5 sec per 



The RECON III is a computer- 
controlled system that uses a Digital 
Equipment Corp. PDP-11/24 programed in 
RSXllM. Peripherals include a color CRT 
and printer. It is capable of 256 remote 
stations. Reporting is by exception 
(report when exceed limits). Each remote 
is capable of 16 analog inputs and has 
16 output control relays (KU series 20A 
relays). Resolution of input signal is 
12 bits (0.024%). The system has a data 
base and graphics edition and a capabil- 
ity to download to remotes over the data 
line. Remotes can scan subremotes. 

A microprocessor-based system, MIC- 
RECON, is under development and will be 
released soon. 

Transmitton Ltd. 
Smisby Road 
Ashby-De-la-Zouch 
England LE6 5UG 
0530-415941 

Reliability Technology 
150 Plum Industrial Court 
Pittsburgh, PA 15239 
(412) 325-3121 

Transmitton Ltd. is also qualified 
to manufacture the British MINOS system. 



The Transmitton control consol consists 
of one or two color video displays, a 
keyboard, two switch panels, and a com- 
munications center. The Transmitton sys- 
tem, represented in the United States by 
Reliability Technology, is designed to 
monitor and control the operation of con- 
veyors, pumps, electrical switches, shaft 
elevators, ventilation fans, and bunkers, 
as well as environmental parameters such 
as methane, carbon monoxide, air ve- 
locity, air pressure, temperature, and 
smoke. 

The system consists of a central 
control station and up to 168 outsta- 
tions. Data are transmitted in a digital 
format, but not frequency coded; i.e., 
high-low states are transmitted. A four- 
conductor cable is used. 

Conveyor monitoring includes belt 
speed, misalignment, belt weight, torn 
belt, motor voltage current and tempera- 
ture, vibration, and alarm states. Sonic 
and visual alarms are available locally 
and can be controlled locally or remote- 
ly. Belt start sequencing can be done 
automatically, so that belts can be shut 
down to conserve power. 

Pump monitoring might include 
diagnostics, such as electrical cur- 
rent drain, pressure, flow, etc. Energy 
shedding can be accomplished. Power 
center monitoring can be used to reset 
breakers remotely or to locate a prob- 
lem. Software for summary analysis is 
included. 

The Transmitton systems have been 
installed in hundreds of deep coal mining 
operations, internationally. 

4.5.2b Summary 

A summary of the monitoring sys- 
tems offered by suppliers currently mar- 
keting in the United States is given in 
table 4-3. 



99 



TABLE 4-3. - Mine monitoring system summary 



Aquatrol 


Conspec 


Davis 


Giangarlo 


Hawker 
Siddeley 


Kidde 


1 


4 


1 


1 


7 


2 


98 


128 


127 


512 


16 


64 


18 


1 analog or 
16 digital 
in; 1 dig- 
ital out. 


40 


5 boards 


32 


24 


<18 


1 (16 bit) 


8 


18 


NA 


8 


218 analog 


16 (1 bit) 


32 


'16 


NA 


16 


<6 


NAp 


NAp 


NAp 


NA 


NAp 


2 6 analog 


1 (1 bit) 


4 


'16 


NA 


8 


^1 mi 


44,000 ft 


20 mi 


20 mi 


8 mi 


10 mi 


40-75 


25 


100 


15-23 


NA 


75-100 


5-10 


0.25 


25 


53-7 


NA 


3-4 


MSA 


Mundix 


Outokompu 


R.F.L. 


Sangamo 
Weston 


Trans- 
mit ton 


3 


1 


Unlimited 


1 


32 


6 


38 


128 


32 


1 


8 


28 


^24 


'^16 boards 


20 


8 boards 


32 


28 


8 


'4 


4 


'16 


16 


16 


16 


'8 


16 


'16 


16 


28 analog 


NAp 


'4 


NAp 


NAp 


16 


2 


4 


'8 


16 


'16 


8 


28 


8 mi 


128 mi 


>6 mi 


>10 mi 


32 mi 


10 mi 


25 


80 


5-100 


1.3-1.5 


15-100 


50-60 


1 


3 


3.3 


0.7 


5-15 


4-8 



Trunks 

Outstations per trunk. 
Total input channels 
per outstation. 



Inputs per outstation: 

Analog 

Digital 

Outputs per outstation: 

Analog 

Digital 

Transmission distance. . 
Cost, thousand dollars: 

Central station 

Outstation 



Trunks 

Outstations per trunk.. 
Total input channels 

per outstation 

Inputs per outstation: 

Analog 

Digital 

Outputs per outstation: 

Analog 

Digital 

Transmission distance. . 
Cost, thousand dollars: 

Central station 

Outstation 



NA Not available. 
'Per board. 
2Times 12. 
^Extendable. 

4.5.3 Sensors 



NAp Not applicable. 
"^Typical, 40,000 ft maximum. 
5 Includes CO monitor. 
^575 maximum data points. 



74,096 digital 
maximum. 



and 1,024 analog. 



While mine monitoring systems can 
include both environmental and production 
sensors, this report focuses on environ- 
mental sensors. Of particular interest 
in environmental monitoring are air ve- 
locity, methane and carbon monoxide con- 
centration, and respirable dust. 

Air velocity can be measured by us- 
ing either vane anemometers or acoustic 
vortex-shedding anemometers. Vane ane- 
mometers are basically mechanical devices 



in which the airflow causes the vanes or 
impellers to rotate at a speed propor- 
tional to the airflow. Although they are 
most commonly used underground as direct 
reading, portable instruments, they can 
also be adapted to a mine monitoring sys- 
tem. However, despite the advantage of 
mechanical simplicity, they are suscepti- 
ble to dirt and moisture concentration. 
Vortex-shedding anemometers, on the other 
hand, have no moving parts and use acous- 
tic signals to measure turbulence caused 
by the airflow. Both anemometers have 
the disadvantage that they are fixed 



100 



point measurements normally made in the 
boundary layer near the roof or rib and 
therefore do not necessarily represent a 
true measure of the average airflow. 

Methane and carbon monoxide concen- 
tration can be measured by either heat of 
combustion (i.e., catalytic combustion) 
or infrared absorption techniques. For 
carbon monoxide, electrochemical analysis 
is also commonly used. The heat-of- 
combustion sensor is based on the princi- 
ple that catalytic oxidation (i.e., burn- 
ing) of a combustible gas such as methane 
will result in a temperature rise in the 
sensor in proportion to the gas concen- 
trations. This technique is widely used 
in the methane monitors required for face 
equipment in this country. The infrared 
sensors are based on the fact that dif- 
ferent gases have different infrared en- 
ergy absorption characteristics. These 
sensors have been in use in South African 
and German mines for a number of years. 
Electrochemical analyzers measure the 
carbon monoxide concentration by chemical 
reaction with electrodes that are im- 
mersed in an electrolyte. These sensors 
have been used recently as part of an 
early warning belt fire detection system 
(that in some cases allowed the use of 
the beltway for intake air). 

Remote monitoring of respirable dust 
presents a number of technical difficul- 
ties. Of the various measurement tech- 
niques currently in use, the beta- 
attenuation and optical devices appear to 
be the most suitable for integration into 
mine monitoring systems. The former uses 
beta radiation to detect dust concentra- 
tion, i.e. , the amount of beta absorption 
due to the dust deposited on a sample 
plate is proportional to the dust concen- 
tration. The optical sensors are based 
on the principle that the dust concentra- 
tion is proportional to the amount of 
light reflected by the dust-laden air 
sample. 

The following sections describe the 
sensing or measurement techniques for the 
four parameters of interest that are most 
applicable to underground mine monitor- 
ing. That is, only sensors that are 



suitable for remote, fixed point opera- 
tion underground and that provide an 
electrical output that can be interfaced 
with standard telemetry equipment will be 
discussed. 

4.5.3a Air Velocity Sensors 

There are two basic types of air 
velocity sensors that are applicable to 
underground mining: rotating vane ane- 
mometers and acoustic vortex-shedding 
anemometers. 

The rotating vane anemometers are 
mechanical devices with vanes or im- 
pellers that are rotated or turned by the 
air flowing through the anemometer. The 
better instruments use ball bearings that 
reduce the turning friction of the main 
shaft on which the vanes are mounted to 
improve the accuracy at low air veloc- 
ities. Portable (typically hand held) 
vane anemometers have been a standard air 
velocity measuring instrument in under- 
ground mines for a number of years. The 
Davis vane anemometer is probably the 
most common example of these direct read- 
ing instruments. Recently there has been 
some interest in adopting these in- 
struments to remote monitoring systems; 
for example, American Mine Chemical Co. 
is currently distributing the British 
Abbriko anemometer that provides an elec- 
trical pulse count output proportional to 
the air velocity. However, while the 
device does have certain advantages be- 
cause of simplicity of operation, its 
susceptibility to excessive dirt and 
moisture represents a significant dis- 
advantage. The National Coal Board, ex- 
perimenting with such anemometers (such 
as its BA. 1 and BA.2), found that in- 
creasing the vane diameter increased the 
torque on the center shaft and thereby 
reduced the potential of the shaft seiz- 
ing because of dirt accumulation (9^) . 

The second category of anemometers, 
acoustic vortex-shedding, measure air ve- 
locity by sensing the frequency at which 
vortices are shed from a rod placed in 
the airstream. The vortices, or eddies 
in the airstream, are sensed by the ef- 
fect they have on an acoustic (actually 



101 



ultrasonic) pulse transmitted through 
them. A typical configuration would con- 
sist of a relatively compact package con- 
taining transmitting and receiving trans- 
ducers mounted on opposite sides of a 
small rod and the electronics required to 
transmit the data to the appropriate out- 
station or control panel. Since vortex 
shedding anemometers have no moving 
parts, they are particularly well suited 
for underground mines. However, while 
they are less susceptible to contamina- 
tion than the vane anemometers, they are 
also typically more expensive than the 
mechanical anemometers. 

It should be pointed out that both 
types of anemometers are fixed point 
units and as such have the disadvantage 
of being able to measure the airflow at 
only one point in the airway. This re- 
striction is usually compounded by mount- 
ing the unit close to the roof or rib of 
the airway, i.e., in the boundary layer 
where changes in the average airflow can- 
not always be accurately sensed. Al- 
though there are empirical methods of 
compensating for this measurement defi- 
ciency, they do not always provide the 
most satisfactory solution. 

The two major suppliers of acoustic 
anemometry equipment in this country are 
J-Tec Associates and Mine Safety Appli- 
ance (MSA). The J-Tec model VA-216B is 
approximately 12 by 7 by 4 inches in size 
and can measure air velocity in two 
ranges; 50 to 3,000 fpm and 150 to 10,000 
fpm (±2% of full scale). The unit op- 
erates on a 12 to 21 volts dc at a max- 
imum of 35 mA. While the standard output 
is to 5 volts dc, the unit can also 
provide 1 to 5 mA or 4 to 20 mA as an 
option. Calibration, performed at the 
sensor, is typically recommended at 6- 
week intervals. The price of the unit 
runs between $1,000 and $1,500, including 
output electronics. 

The MSA sonic anemometer can measure 
air velocities up to 25,000 fpm. The 
sensor requires 110 volts ac power (30 w) 
and produces either 0-1 mA or 4-20 mA 
outputs. The list price of the unit is 
between $500 and $600. 



4.5.3b Methane Sensors 

There are two primary techniques of 
detecting and measuring methane concen- 
tration that are suitable for use in mine 
monitoring systems: heat of combustion 
and infrared absorption. Of the two, the 
heat of combustion, or catalytic combus- 
tion, sensors are the most common in this 
country. These sensors detect the pres- 
ence and concentration of methane by mea- 
suring the temperature rise of a cata- 
lytic element that oxidizes (i.e., burns) 
the methane at very low temperatures 
without a flame. The temperature rise in 
the catalyst is proportional to the meth- 
ane content of the air surrounding the 
sensor. 

There is some difference in the 
technique by which the sensors expose the 
catalyst to the gas mixture to be mea- 
sured. Some devices rely on diffusion of 
the gas mixture through a porous metal 
flame arrestor screen. These are often 
referred to as "diffusion-head" type sen- 
sors. Others use mechanical pumps to 
drain air samples across the catalyst. A 
third method, referred to as "sniff and 
sneeze," alternately draws the sample in 
and then exhales prior to the next sam- 
ple. While the diffusion devices have a 
slower response time they are simpler and 
do not rely on mechanical pumps that may 
be affected by dust and moisture. 
Diffusion-type methane sensors are typi- 
cally used in the monitoring and auto- 
matic deenergizing devices required on 
face equipment in U.S. coal mines. Al- 
though catalytic combustion sensors are 
relatively rugged and simple in opera- 
tion, they do have (at least in princi- 
ple) a disadvantage in terms of specific- 
ity. That is, the catalyst temperature 
will rise in the presence of any combus- 
tion gas, not just methane. However, 
this disadvantage is not always a major 
problem and can be reduced somewhat by 
operating at a specified temperature or 
selecting a catalyst that favors a 
methane-oriented chemical reaction. A 
second, and possibly more important, dis- 
advantage is that catalytic sensors are 
not generally suitable for measuring 
methane concentrations above 5%. 



102 



The second methane sensing technique 
is based on the absorption, by different 
gases, of different amounts of infrared 
radiation. In a typical configuration, 
infrared energy is passed through a sam- 
ple cell that has windows that do not ab- 
sorb in the infrared band. Either the 
sensor is equipped with a reference cell 
or the sensor is calibrated by purging 
the sample cell with nitrogen prior to 
making any measurements. 

An infrared detector, located on 
the opposite side of the cell, produces 
an electrical signal proportional to 
the difference between the reference and 
the sample. This signal is, in turn. 



proportional to the methane concentra- 
tion. Infrared sensors can be used to 
measure methane concentrations in the en- 
tire range between 0% and 100%. While 
these devices are relatively sensitive 
and specific, they are typically more 
complex and expensive than the catalytic 
sensors. 

Table 4-4 summarizes the methane 
sensors currently available in this coun- 
try. Although only one infrared sensor 
is mentioned, it should be noted that 
several infrared sensors have 
veloped in other countries, 
the South African SPANAIR and 
UNOR. 



been de- 
among them 
the German 



TABLE 4-4. - Methane sensors 



Cost' 



Company 



Model 



Measuring 
principle 



Range, 

% 



Electrical 
output 



Power 
requirements 



Appalachian 

Electronics. 
Bacharach 

CEA 

CSE 

Dynamation 

ERDCO 

GasTech 

General Monitors... 

J-Tec 

MSA 

NMS 

Scott Aviation 

Texas Analytical... 
NA Not available. 



102A 
CD800 

RI550A 
140 

1210EX 

250 

1620 



NA. 



480 



VMIOIB 



1810-0073 

40008561 
40008015 

1930B 



Catalyst 

Infrared 
Catalyst 

. . .do. . . 

. . .do. . . 

. . .do. . . 

. . .do. . . 

. . .do. . . 

. . .do. . . 

. . .do. . . 

. .do. . . 
. .do. . . 



0-99 

0- 5 

0- 2 
0- 5 

0- 5 

0-50 

0- 5 

0- 5 

0- 5 

0- 5 

0- 2 

0- 5 

0- 5 
0- 5 



Digital 
0-100 mV 

0-10 mV 
0-100 mV 

0-1 mA 

0-50 mV 
0-100 mV 

0-100 mV 

4-20 mA 

0-5 V dc 

0-1 V dc 
4-20 mA 

4-20 mA 

4-20 mA 

0-1 mA 
4-20 mA 



110 V ac 
110 V ac 

110 V ac 

110 V ac 
270 V dc 

110 V ac 
12 V dc 

110 V ac 
24 V dc 

110 V ac 
12 V dc 

110 V ac 
24 V dc 

12-21 V dc 

110 V ac 

12 V dc 

24 V dc 

110 V ac 
12-24 V dc 



$1,800 
1,100 

2,200 
2,200 

NA 

1,200- 
3,000 

1,100 

1,200 

500- 
1,000 

500 

300 

400 
1,400 



'Rounded to nearest $100. 



103 



4.5.3c Carbon Monoxide Sensors 

There are three techniques of car- 
bon monoxide sensing that may be con- 
sidered suitable for use in mine moni- 
toring systems : electrochemical reac- 
tion, catalytic-combustion, and infrared 
absorption. 

Electrochemical sensors contain a 
sensing electrode, a counter electrode, 
and sometimes a reference electrode in an 
electrolyte (such as sulfuric acid solu- 
tion for the Energetic Sciences Ecolyzer 
2000) . The air to be sampled either is 
allowed to diffuse into the sensor or is 
drawn in by a mechanical pump. The car- 
bon monoxide in the air reacts with the 
electrodes, generating an electric signal 
proportional to the carbon monoxide con- 
centration in the air sample. General 
Electric Co. has developed, in conjunc- 
tion with the Bureau of Mines, a fuel 
cell carbon monoxide sensor that uses a 
solid polymer electrolyte. This design, 
of course, has the advantage of not hav- 
ing a liquid electrolyte that can spill. 



IS 



The catalytic combustion technique 
quite similar to that discussed 



earlier for methane sensing. That is, 
the air sample is oxidized in the pres- 
ence of a catalyst, with the resulting 
temperature rise in the catalyst being 
proportional to the gas concentration. 
As mentioned earlier, the basic technique 
is nonspecific, and carbon-monoxide -spe- 
cific catalysts and appropriate filament 
temperatures are required to reduce the 
interference of other combustible gases. 

The infrared technique is also simi- 
lar to that described for methane detec- 
tion. The sensor is made selective for 
carbon monoxide by modifying the receiver 
transducer to detect changes in the in- 
frared wavelengths that are absorbed by 
carbon monoxide molecules. For a nondis- 
persive system, the absorption filters 
must be changed, and for the dispersive 
systems, the refraction grating may have 
to be changed. 

Table 4-5 lists several representa- 
tive carbon monoxide sensor suppliers. 
While no infrared carbon monoxide sensors 
are listed, at least one is manufactured 
in South Africa. It is called the 
SPANAIR, and it can also be modified for 
use as a methane detector. 



TABLE 4-5. - Carbon monoxide sensors 



Company 



Model 



Measuring 
principle 



Range , 
PPm 



Electrical 
output 



Power re- 
quirements 



Cost 



Dynamation. 



Energetic Sciencies 



General Electric. 



MSA. 



Neutronics. 



CO-2300 
CO-300 

4125 
15ECS6 
571 
910 



Catalyst 

. . .do 

Electrochemical 



Fuel cell. 



Electrochemical 



.do. 



0- 300 
0- 300 

0- 50 

0-1,000 

0- 100 
0- 500 

0-4,000 



0-1 mA 
0-1 mA 

0-1 V dc 
0-20 mA 
dc 
4-20 mA 



} 0-1 V 



117 V ac 

110 V ac 

12 V dc 

110 V ac 

14-28 V dc 

7-38 V dc 

115 V ac 

19-60 V dc 

110 V ac 



$950 
925 

1,700 

NA 

1,830 

1,200 



NA Not available. 



104 



4.5.3d Dust Sensors 

Although there are many types of 
techniques to measure dust concentrations 
in mine air, a number of technical diffi- 
culties limit the availability of dust 
sensors that would be suitable for use in 
automatic, remote, mine monitoring sys- 
tems. For example, while the Bendix-type 
personal dust sampler has been used suc- 
cessfully for a number of years, it is 
not amenable to remote monitoring because 
the filters must be manually removed, 
weighed, and replaced. Three measurement 
techniques that might be adopted to re- 
mote monitoring are described below; they 
are optical sensing, piezoelectric sens- 
ing, and beta attenuation. 

In optical sensors, a beam of light 
(from either an incandescent or laser 
source) is directed into a chamber that 
contains a sample of the dust-laden air. 
The intensity or brightness of the light 
that is scattered by the dust cloud in 
the chamber is governed by the surface 
area of the dust particles. The inten- 
sity of the reflected light is typically 
determined by comparison with a portion 
of the direct light. While the device is 
normally manual in operation (the di- 
rect light is passed through a variable 
filter that is adjusted until the inten- 
sity of the direct light equals the re- 
flected light), at least one company man- 
ufactures an optical dust monitor with 
an electrical output. The Japanese-made 
Horiba monitor provides an output of to 
20 mA, but since it requires 110 volts 
ac (10 W) , MSHA approval is necessary if 
it is going to be used in a return air- 
way. Another unit, GCA model RAM-1 dust 
monitor, measures concentrations in the 
range of 1 to 200 mg/m^ , has a 0- to 10- 
volt dc output and runs on 110 volts ac. 
The cost of this unit is approximately 
$6,000. 

The main disadvantage of this tech- 
nique is that a direct comparison between 
dust concentrations by this technique and 
other methods is possible only when the 
particle size distribution of the dust in 
the air sample is the same as that used 
for instrument calibration. 



The second method is piezoelectric 
sensing. In this type of dust sensor, 
particles are drawn through an orifice 
and deposited on the face of a quartz 
crystal. This crystal is part of an 
oscillator whose resonant frequency 
changes linearly with small changes 
in crystal thickness (or mass). As 
particulate mass collects on the crys- 
tal face, the frequency decreases. 
Therefore, the rate of frequency change 
is proportional to the airborne mass 
concentration. 

The third technique is beta attenua- 
tion. In beta attenuation instruments, 
the aerosol is drawn through an orifice, 
and particles impact on a suitable sur- 
face. The impact surface is positioned 
between a beta radiation source and a 
counter. The amount of beta absorption 
recorded by the counter is proportional 
to the dust concentration. The major ex- 
ample of this technique is the GCA high- 
concentration dust monitor that GCA de- 
veloped in conjunction with the Bureau of 
Mines. 

The major advantage of the GCA dust 
monitor over the light-scattering moni- 
tors is that (within certain limits) the 
GCA unit measures the mass concentration 
independent of the type of dust and par- 
ticle size distribution. 

4.6 Existing Mine Monitoring Systems 

Monitoring systems have been in- 
stalled in a limited number of U.S. 
coal and metal and nonmetal mines. In 
addition, numerous systems are installed 
in foreign countries. In this section, 
several installations are described; 
the reasons for their development and 
comments that are relevant to the cen- 
tral issue of monitoring and/or control 
in underground coal mining are given. 
In cases where the data have been pub- 
lished and are available, the name of 
the mine is listed. In cases where 
data were obtained from mine person- 
nel and are not generally available, 
only a letter designation for the mine is 
provided. 



105 



4.6.1 U.S. Undergound Coal Mines 

Mine A 

Coal producer A operates a coal mine 
in support of its steelmaking operations. 
A very extensive monitoring and data ac- 
quisition system was installed in 1978. 
The system has production, management, 
maintenance, and safety components. 

In 1978, production was lagging and 
morale was poor. A human relations audit 
indicated that section foremen and others 
felt isolated and left to fend for them- 
selves. Communications were poor. Sup- 
port to section foremen was perceived as 
slow and often ineffectual. 

The system that was installed fo- 
cused on improved communication and back- 
up support. It monitors production, pro- 
vides maintenance support from a data 
base system, supports underground manage- 
ment with improved reporting and communi- 
cation, and enhances safety with tracking 
and followup of unsafe conditions. 

At present, phone reports are re- 
quired from each section foreman every 2 
hours (soon to become every hour when a 
leaky coaxial radio link system becomes 
operational). The status reports are re- 
ceived by an operator who codes the 
status, including mechanical problems and 
repair effort underway, onto a data base 
reporting system. These reports include 

Conditions on section. 

When loading of coal started. 

Mechanical delays. 

Nature of delay. 

Start-stop time. 

Mechanical problems are keyed to ma- 
chine and location. Information is coded 
by a communications coordinator, then 
telemetered over phone lines to a central 



office computer. Since a communications 
coordinator is the only one to enter the 
data, it is coded in a uniform fashion. 

In turn, section foremen are able to 
interrogate the system at any of the 
three underground communication centers 
(one at base of portal, and one each at 
East and West mains). 

Spare parts inventory software was 
generated from an existing program used 
in the mill activity. A detailed, pic- 
torial blowup with parts number callout 
is available near each section. As a re- 
sult, the inventory status, location, and 
ordering (reorder from vendor, if neces- 
sary) can be accomplished quickly under- 
ground. Similarly, management can track 
problems by interrogating the system. 

Because the bihourly reports are 
relatively complete, formal end-of-shift 
reports are no longer necessary, and on- 
going foremen have a current status sheet 
on which to base their plans. 

This mine installed the cable for 
the system, purchased modems to enable 
communication of the data over phone 
lines , and then leased the remainder of 
the system, including dedicated phone 
lines to the computer facility, line 
printers, CRT displays, etc. 

Software was developed in-house, al- 
though some of it already existed, since 
it was in use in the associated steel 
mill. Software development times were 
estimated to be 



Production system.... 
General underground.. 

Maintenance 

Safety 



14 man-months. 

18 man-months, 

Existed. 

12 man-months. 



Surprisingly, one of the most trou- 
blesome links in the system has been the 
leased commercial phone lines (2,400 
baud) used to telemeter data to the cen- 
tral computer facility. There have been 
instances of outages for days. 



106 



Mine B 

Coal producer B has a monitoring 
system that uses equipment built by Larse 
Corp. of Palo Alto, CA. It is a hard- 
wired, time-domain multiplexed system 
(channels interrogated sequentially in 
time), designed originally for building, 
process control, or energy monitoring. 
The producer uses the system for belt 
monitoring and for carbon monoxide and 
temperature monitoring on the slope (the 
mine has a propane preheat system) . 

The history of the system is that in 
1975, local management personnel had de- 
cided to upgrade the telephone system to 
a dial-phone system. Their chief elec- 
trical engineer urged simultaneous in- 
stallation of a monitoring capability, 
perhaps integrated with the new phone 
system. He obtained mine managers' ap- 
proval, and invested approximately one- 
third of his time during the next 3 
years selecting, purchasing, installing, 
and testing the system. He was assisted 
underground with cable stringing and 
other installation tasks about one-sixth 
of the time. 

The Larse system that was purchased 
is capable of transmitting only binary 
data. The sequence time is 3 sec per 
station. Transmission is over a spare 
twisted pair of conductors in the mine 
phone cable. An independent electronics 
and manufacturing firm developed and fab- 
ricted solid-state interface circuitry 
designed to convert voltages on control 
switch terminals to logic levels compati- 
ble with the Larse system. The converter 
was optically coupled so that belt se- 
quencer circuitry is uncoupled from the 
monitor, and the converter has no moving 
parts, enhancing reliability. A printer 
is used to generate hard copy of alarm or 
status change. A light panel is also 
used to display status. 

Mine personnel are evaluting the de- 
sirability of installing a J-Tec velocity 
and methane system. Their most likely 
application would be a continuous monitor 



of worked out panels, eliminating the 
need for a 4-hour inspection mandated by 
the State. 

Extensions of this system might in- 
clude ventilation control. In that case, 
a variable-speed synchronous motor drive 
on their fan might be used to reduce 
energy demand when the full output is not 
needed. At present, 1,600 hp is drawn 
for ventilation. Original plans called 
for some airflow and methane monitoring, 
but this step was deferred as "too expen- 
sive with little payback." 

The monitoring system is maintained 
by one man, who also maintains the tele- 
phone system (which requires much more 
time than maintaining the monitoring 
system) . 

Mine C 

Operator C has several mines that 
have very gassy seams and heavy over- 
burden with difficult roof control prob- 
lems. As a result, personnel at this 
mine have received a variance from MSHA 
allowing the use of beltways as an inlet 
aircourse. 

They have 5 years of experience with 
carbon monoxide monitors, using Energetic 
Sciences sensors. The current Energetic 
Sciences monitors that are purchased have 
two level alarms and a built-in battery 
pack for uninterruptable power supply. 

The telemetry system is an S.R. 
Smith system that transmits digital in- 
formation only and necessitates a dual 
threshold modification to the Ecolyzer. 
False alarms are generated with the 
Ecolyzer as the result of power intermit- 
tencies, a prime factor in the insistence 
on an uninterruptible battery supply. To 
enhance reliability, redundant PDP-11/4 
computers have been installed to analyze 
and display data. Maintenance by DEC has 
sometimes been slow, although the system 
has been completely down only once in the 
last 2 years , thanks primarily to redun- 
dant equipment. 



107 



Plans include staffing up to one 
full-time engineer per mine for the moni- 
toring system. Although the system ini- 
tially gave frequency "nuisance" alarms, 
those problems have largely been solved, 
with the occasional exception of false 
alarms associated with the monthly cali- 
bration procedure. They also conduct a 
weekly inspection. Where alarms are dis- 
played at the surface, the appropriate 
site is notified by telephone. 

Future plans include monitoring of 
methane and airflow in returns. They 
have a methane drainage research project 
for which they have currently purchased 
methane and airflow monitors. During a 
6-week, period in the winter of 1981, they 
lost 10 shifts of longwall production as 
a result of gas outs , shutdowns that are 
sometimes mine wide and are the motiva- 
tion for their ventilation monitoring and 
control plans. 

Adjustment and deployment of the 
monitoring system hardware, including 
sensors, is a nuisance factor. There 
have also been incidents of either will- 
ful or careless destruction of equipment, 
particularly sensors. 



two mines are equipped with belt fire 
monitoring equipment and two more are be- 
ing outfitted. The two that are present- 
ly on-line have been operating for about 
6 months. The system consists of Ecolyz- 
er carbon monoxide sensors, with up to 40 
monitors per mine, a Giangarlo transmis- 
sion system, and Niagra Scientific compu- 
tational and display equipment. There is 
no maintenance contract. The software 
was provided by Giangarlo. The system 
scans stations at a rate of about 1 per 
second. The cabling for the system is 
military surplus wire that is four- 
conductor twisted No. 19 wire with steel 
armor, which mine personnel purchased in- 
expensively. It has been found that the 
digital (frequency shift) signals can be 
transmitted up to about 3 miles with vir- 
tually no problems. The alarm algorithm 
at present is simply a level exceedance 
alarm. There have been four cable fail- 
ures in the last 6 months, two failed 
open and two failed shorted, owing to ac- 
cidents with large boulders on the belt. 
In the case of cables failing open, the 
system continued to operate up to the ca- 
ble break, and in the case of cabling 
failing shorted, the entire system was 
faulted until repaired. 



They also monitor bearing tempera- 
ture and water gage on their main ven- 
tilation fans, using a FSK audio band 
t e lame try s cheme . 

The software for the S.R. Smith sys- 
tem was written by a software specialist. 
The costs for software were viewed as 
substantial. 

Mine D 

Coal producer D has made a very sub- 
stantial commitment to its mine-wide 
monitoring, data acquisition, and con- 
trol. Its mines are deep, with typically 
more than 2,000-foot overburden, are very 
gassy, and have difficult roof control 
problems. As a result, the producer has 
received permission to use the belt pas- 
sageway for inlet air, provided that 
carbon monoxide is monitored. Currently 



It is estimated that maintenance 
of the system requires about two man- 
shifts per week, including daily, week- 
ly, and monthly inspections. The daily 
inspection is a physical inspection by 
the fire boss, the weekly inspection in- 
volves electrical tests, and the monthly 
inspection involves span calibration. 
The maintenance effort was estimated to 
be approximately one-half inspections 
and calibrations and one-half repair 
functions. 

Ac power tends to be unreliable and 
intermittent. As a result, a gel-cell 
power has been installed for each sensor 
with a 48-hour capacity and continuous 
trickle charge. In the event of inadver- 
tent disconnect and resultant battery 
discharge, the Ecolyzer indicates a false 
alarm. 



108 



All four systems were expected to be 
operating by 1982. Following completion 
of this carbon monoxide fire monitoring 
system, the system will be extended to 
monitor belt operation. In the No. 3 
mine, there are 25 belts, each typically 
4,000 feet long. The intent of the belt 
monitoring is to display remotely infor- 
mation about which belts are down and 
possibly, diagnostics as to failure mode. 

Because the seam being mined is ex- 
tremely gassy, the required amount of air 
is very high. This demand, combined with 
the extreme depth, results in very high 
water gage and very high ventilation 
costs. As a result, variable pitch ven- 
tilation fans have been installed, and 
methane and airflow will be monitored in 
the returns and the fan pitch modified 
accordingly to reduce ventilation costs. 
When a 2,000-hp fan was increased to a 
larger 7,000-hp variable-pitch fan, the 
power costs increased $1,900 per day at 
that mine. Plans also exist to monitor 
the temperature and vibration of fan 
bearings. 

In addition, the producer is also 
automating its billing, inventory con- 
trol, and maintenance functions, such as 
maintenance history, equipment inventory, 
lubrication histories, preventive mainte- 
nance, etc. , as well as automating some 
mapping and plotter routines. These com- 
putational facilities will use their own 
computers and programs and not piggyback 
on the fire monitoring system. 

Other monitoring plans include 
production tonnage monitoring, by sec- 
tion and shift, as well as power cen- 
ter monitoring, particularly on longwall 
sections. 

Mine E 

Mine monitoring at operation E has 
the following three distinct functions: 

Trapped miner location-roof fall 
monitoring. 



Ventilation monitoring at 11 
stations. 

Fan monitoring. 

Trapped Miner-Roof Fall Detection 

The trapped miner system was origi- 
nally installed several years ago as a 
Bureau-funded research project. Six com- 
mercial, 40-Hz resonance, moving coil 
geophones are implanted about 2,000 feet 
apart in 50-ft-deep holes backfilled 
with sand. Their seismic output is fed 
to a Texas Instruments 980A computer that 
computes the epicenter of any event ob- 
served by three or more geophones. The 
software package includes graphics that 
plot the trapped miner-roof fall location 
with a triangle if three geophones 
received the signal, a square if four 
were activated, and an asterisk if five 
or more sensed the disturbance. The co- 
ordinates and computed confidence margin 
(error) are printed. Triangulation in 
the southern mine sections where the 
equipment is located is to within 50 to 
100 feet. A disturbance count is also 
recorded. The trapped miner equipment is 
all located on the surface. 

Ventilation Monitoring 

Mine personnel at operation E were 
interested in upgrading mine technology 
and therefore were receptive to a Bureau 
of Mines request for a cooperative devel- 
opment. An electrical engineer from re- 
search was assigned to design and super- 
vise the fabrication and installation of 
a ventilation monitoring (methane, carbon 
monoxide, air velocity, temperature) sys- 
tem and to assist in the development of a 
trapped miner location system. The 
telemetry system he designed is an audio 
frequency, FSK serial transmission sys- 
tem. It has a capacity for 16 stations 
and a capability for up to eight analog 
inputs at each station (0-5 volts dc). 
Data are telemetered in an audio band. 
The system is powered with a converter 
that uses power from the trolley wire. 



109 



converting -345 volts dc trolley power to 
13 volts dc. At each station this 13- 
volt power is again converted to ±15 and 
±5 for logic and control circuits. Meth- 
ane is detected by mounting a Bacharach 
cell directly on top of the explosion- 
proof container that houses the elec- 
tronics. Access to the box is via multi- 
pin Amphenol environmental connectors. 

Carbon monoxide is detected with an 
Ecolyzer 4000 device, from which the 110- 
volt converter has been removed. Air 
velocity is measured with a J-Tec ane- 
mometer, mounted on a 3-ft-long pole near 
the center of the passageway. Flow cali- 
bration is accomplished with a Davis 
anemometer. 

The processing display package 
prints hourly summaries of the low read- 
ing, high reading, and hourly average at 
each station. 

Manual cross-checks of the system by 
the fire boss on his round are used to 
verify system accuracy and reliability. 
Each station is interrogated electroni- 
cally every 5 min. The system has a 
capacity for 16 stations and a maximum 
cycle rate of 2.5 seconds per station. 

Telemetry cables (unshielded twisted 
pairs) are strung adjacent to the trolley 
wires. The operation is gradually relo- 
cating these cables, since the most fre- 
quent failure mode for the system is a 
loss of telemetry because of a cable cut 
by a derailed trolley pantograph. Be- 
cause of line losses, the sensors must be 
within 500 feet of the power supply and 
telemetry system. 

Data are processed at the surface 
(thresholding, time averaging, high and 
low peak) and stored on a Phillips cas- 
sette. The cassettes can be processed 
with a print routine to generate hard 
copy. 

A problem encountered during devel- 
opment of the wire power-to-12-volt dc 



converter was that there are massive 
transients on the trolley wire that the 
converter could not handle. The solu- 
tion was to use a high wattage voltage 
divider with a zener limit, then dc-to- 
dc convert the voltage-divided, filtered 
dc. 

The operation has asked for exten- 
sion of the ventilation monitoring system 
to the north to cover the other half of 
the mine. However, the research staff is 
now committed to other projects. 

All maintenance and operation is 
performed in-house. Spare boards are 
stocked for the TI 980 and the computer 
is maintained by swapping out defective 
boards. Normal maintenance and record 
keeping takes about 45 min per day. The 
system costs a little over $100,000. In- 
dividual stations cost $2,000 to $2,500. 
The biggest maintenance cost, by far, is 
line repair. 

Fan Monitoring 

The primary ventilation fans are 
continuously monitored. Local system dc 
power and fan head, in inches of water, 
are reported hourly with a low, average, 
and high reading. Alarm status is, of 
course, immediately printed. 

Mine F 

Operator F has three mines that 
share a power distribution system and 
that have very substantial monitoring in- 
strumentation. Mine personnel have also 
begun development of a data base acquisi- 
tion system. The system consists of sev- 
eral layers, developed sequentially. 

The initial installation was a sur- 
face system for fans and circuit break- 
ers. Later, haulage monitoring and un- 
derground fire detection were added. A 
large data base supervisory control 
system is now being installed that will 
be used to upgrade the fan monitoring 
to include vibration and temperature 



110 



monitoring so as to (1) anticipate fail- 
ure, (2) detect smoldering fires by us- 
ing carbon monoxide monitors, (3) con- 
trol utility costs by power shedding when 
desirable, (4) extend haulage monitor- 
ing and control, and/or (5) acquire and 
store maintenance data on machines for 
preventive maintenance and failure 
analysis. 

Quotations for this system are being 
solicited from mine monitoring system 
vendors and particularly from process 
control and energy management equipment 
vendors. A brief summary of the equip- 
ment to date follows. 

Surface System 



ing given to power center control and 
load shedding. At present, overall power 
drain data are provided to dispatchers, 
so that they can shed "unnecessary" or 
lower priority loads; however, shedding 
rarely happens because of production 
pressures and the overwhelming complexity 
of the problem. Effective control re- 
quires mine-wide information, analysis, 
and synthesis of the data. 

The operation buys power from the 
local utility at 69 kV, then distributes 
and transforms the power for the opera- 
tions. Power billing is in proportion 
to the highest 30-min average, using 
sequential time windows (not a floating 
window) . 



The surface system monitors ventila- 
tion fans and the status of circuit 
breakers. At present, there are 13 fans 
in the complex and 28 monitoring substa- 
tions. The system was developed in-house 
in 1973 and uses a hard-wired FSK tech- 
nique, with a 40-channel capacity. 

Underground Haulage and Fire Detection 

At present there are about six sec- 
tions of instrumented haulage. A CRT 
display at the dispatcher indicates the 
number of available empty cars at each 
station, an estimated time of depletion 
of cars (a function of whether the mine 
is cutting), the number of loaded cars, a 
count of loaded cars for the shift, and 
whether the belt is running. 

Heat sensor fire detection outputs 
are also displayed. Data acquisition 
is accomplished with a Westinghouse 
neumalogic system. A deluge system is 
controlled by the output. 

Power Monitoring 

The three mines have a monthly util- 
ity bill of several hundred thousand dol- 
lars. As a result, attention is being 



Methane Monitoring 

Methane is continuously monitored in 
one abandoned area. 

Mine G 

Operator G has a Motorola Intract 
2000, UHF high-band transmitter-receiver 
system that is used at three mines. Per- 
sonnel at this mine monitor ventilation 
fan operation, sensing water gage, and 
measuring bearing temperature and vibra- 
tion. In each instance, the alarm level 
exceedance is the only data transmitted. 
In the case of circuit breakers, they 
have the capability to reset, trip, and 
monitor each of these. 

Each mine typically has several fans 
and perhaps 20 circuit breakers physical- 
ly distributed over the countryside, per- 
haps up to 4 or 5 miles distant from the 
portal. Electrical power is either 7,200 
volts ac three-phase or 300-volt dc trol- 
ley power. In the case of circuit break- 
ers, the system is used primarily for 
control and for resetting after interrup- 
tions. They reset as a test, and, if 
there is a second interruption, they dis- 
patch a man to the site. Since fan 



Ill 



monitoring is required by law, the system 
saves personnel assignment to monitor the 
remote fans. 

At the central station, located in 
the maintenance shop, data are displayed 
on a console with eight status lights, 
designating the presence or absence of 
threshold exceedance of any one of eight 
sensed variables at a site. There is al- 
so a hard-copy printout on a Tl printer 
of twice-a-day status, plus alarms. One- 
third spares are maintained, and Motorola 
gives a 3-to-8-week turnaround time for 
repairs. The system has been on-line for 
15 months, with many initial problems 
that have evidently been worked out. 

Two fully qualified technicians 
with FCC licenses are on the staff and 
a third is being sought. Testing and 
qualification certification is contracted 
out. 

Monitoring of this sort dates to 
1969 when it began with a FEMCO hardware 
system. In 1975, an intermediate radio 
system was added and, later, the Infract 
2000. 

The parent research organization has 
also experimented with the feasibility of 
monitoring cars, as well as production 
activity, on each section, to improve the 
efficiency of the dispatch of cars to 
operating sections. This was a test of 
instrumentation and technique, using one 
section. They measured 

1. Is the section operating? 

a. Continuous miner off. 

b. Continuous miner tramming. 

c. Continuous miner cutting. 

2. At the section, how many — 

a. Cars at ramp. 

b. Empty cars waiting. 



c. Full cars. 

d. Empty — full conversions. 

3. How many cars available at the 
dump site? 

The goal was to reduce the "no empty" de- 
lays at the section. 

The dispatcher has voice communica- 
tion with the locomotive engineer and the 
loading supervisor at each section. De- 
velopment of the design began with the 
following constraints: 

1. Commercial sensors to be used, 
perhaps repackaged. 

2. Existing phone lines to be used 
(Gaitronics) [but not same wires, i.e., 
use phone-quality wiring. Cable is fig- 
ure 8, wire-supported, with 40 conductors 
in pairs. ] . 

The sensors tested were 

1. Infrared photocell (car count). 

2. In-track magnetic proximity 
(count) . 

3. Sonic level (full-empty). 

4. Current sensor at load center. 

a. Off 

b. Tramming 

c. Cutting 

5. Traffic switch positions. 

All data were telemetered to the 
dispatcher. There are local displays for 
debugging purposes. Intel microproces- 
sors were used in remote stations. 

A test of the sensing system was 
conducted by stationing an observer to 
note visually car passage, operating 



112 



time, empty-full status, and switch posi- 
tion. The test results were 



Visual Automatic Error 



582 


599 


3% 


230 


233 


1% 


820 


813 




49 


49 






Direction 

Empty-full 

Operating 

time min. . 

Switch position 



It was concluded that sensing reliability 
was good and quite adequate. 

The most difficult problems were (1) 
the deployment-redeployment of sensors, 
and (2) cost-complexity of the system. 

It was concluded that this type of 
rail haulage monitoring is technically 
feasible; however, the economic viabil- 
ity is probabilistic. There is econom- 
ic payback only if all sections are 
instrumented, since intersection co- 
ordination is the goal. The operator 
opted for a computer simulation at this 
point to evaluate the economics, using 
a modified version of the Penn State 
RailSim program. The results indicated 
substantial incremental improvement in 
productivity, but since the operation 
is market limited, the economics for 
such a system were not quite attractive; 
i.e., it cannot easily sell more coal per 
year and cannot capitalize on the poten- 
tial to run less days per year because 
of large capital costs and institutional 
factors. 

A simpler less costly system might 
be viable. For example, a system that 
reported 

1. Whether the section is operating 
(power center). 

2. What the car count is at the 
dump site. 

3. Other data manually input by the 
dispatcher. 

Since use of longwall sections is 
expected soon, problems with haulage 



coordination will be intensified. "In- 
tangibles" (safety, morale, health, labor 
resistance, sabotage, etc.) are hard to 
evaluate. 

A "smart sensor" standalone is being 
considered that — 

1. Obtains power off rail line. 

2. Senses a parameter, converts 
data to a secure format. 

3. Transmits wireless. 

Passive (unpowered) sensors are used 
wherever possible, and sensors are mini- 
mized (more manual input). 

Power Monitoring 

Billing of power used by energy- 
intensive industrial users is based on a 
base rate plus a peak load factor. Mine 
G's local utility expects to go to a peak 
factor based on the highest 5-min aver- 
age, perhaps highest floating 5-min aver- 
age. Although the potential savings are 
very substantial, an automatic power 
management system is clearly required to 
react within 5-min windows. 

A very ambitious computerized main- 
tenance management program has also been 
begun. There seems to be a likelihood 
that haulage and ventilation monitor- 
ing and/or control will eventually be 
attempted. 

Mine H, Federal No. 2 

Sensors for carbon monoxide and 
methane were placed so as to compare 
methane concentrations entering and leav- 
ing each section. Differential pressures 
were also measured to highlight ventila- 
tion problems owing to partial blockages, 
etc. Temperature, humidity, and air 
velocity were also measured at dozens of 
sensor stations. Computer control was 
performed at West Virginia University. 
These early tests served to prove that 
the task of underground monitoring was 
technologically feasible (2). 



113 



More recently, a similar system has 
been used to test the concept of ventila- 
tion control (1). Ventilation param- 
eters, including methane, carbon monox- 
ide, temperature, humidity, airflow, and 
differential pressure, were measured. 
Ventilation regulators (louver door de- 
sign) were operated electronically, and 
the resultant ventilation redistribution 
was monitored. 

Mine I 

Drainage of methane in advance of 
mining is an effective way of minimiz- 
ing methane content at the working face. 
Since most coal mining countries have 
enacted safety regulations that require 
that face equipment be shut down if the 
methane content in the air reaches a 
predetermined level, methane drainage 
has production as well as safety bene- 
fits. Because methane liberation in- 
creases with rate of coal production, 
methane drainage is more desirable in 
highly mechanized longwall operations 
where production rates are relatively 
high. As a result, methane drainage is 
used extensively in Europe where longwall 
mining has become very common. In 1975, 
approximately 7.5 billion cubic feet of 
methane was drained in Polish mines , 
and over 21 billion cubic feet were 
drained from mines in the Federal Repub- 
lic of Germany (7). Recently, methane 
drainage has also become more important 
as the production rates in U.S. mines 
have increased. 

Because of the inherent design that 
roof falls, bottom heaving, etc., may 
rupture the methane drainage piping, 
the Bureau of Mines has specified 
that drainage systems be equipped with 
'a "fail-safe" monitoring and control 
system (22) . Basically, the system 
should be able to shut off the flow of 
methane into the drainage pipe in the 
event of unsafe conditions such as a 
power failure or break in the drainage 
piping. 



One such system has been success- 
fully employed in the Bethlehem Mines, 
Marianna No. 58, located in Marianna, Pa. 
This system consists of MSA methane sen- 
sors located at 500-foot intervals in the 
airway containing the methane drainage 
pipe. The output from these intrinsical- 
ly safe sensors is transmitted to a con- 
trol panel located in a nearby fresh air 
entry. Also located in the fresh air 
entry is a small (1.05-cfm) air compres- 
sor that supplies air through PVC tubing 
strapped to the drainage pipe, to the 
pneumatically actuated shutoff valves lo- 
cated at each borehole. The valves are 
spring loaded and held open by the air 
from the compressor. 

The system can also incorporate 
a "receiving" station (located above 
ground) that contains meters that in- 
dicate the methane content measured by 
the sensors and recorders that provide a 
continuous log of the sensor output. 

In the event of a power failure, the 
compressor would cease to function, the 
air pressure in the PVC tubing (normally 
55 psig) would drop, and the valves would 
close. If the methane content in the 
airway containing the drainage pipe ex- 
ceeds 1% (due to a leak in the pipe) , the 
PVC tubing is vented by solenoids located 
on the control panel and the valves again 
shut down the methane flow. In addition, 
if a roof fall ruptures the drainage 
pipe, it will also rupture the other PVC 
tubing that is strapped to the top of the 
pipe, automatically dropping the pressure 
and closing the valves. Finally, if a 
sensor fails to operate or a cable is 
severed, the solenoids will vent the PVC 
tubing and again close the borehole 
valves. 

References 10 and 18 discuss this 
system in more detail. 

A summary of the monitoring systems 
reviewed is presented in table 4-6. 



L14 



TABLE 4-6. - Summary of monitoring systems surveyed in U.S. underground coal mines 





A 


B 


C 


Function. ............ 


Production, maintenance, 

communication. 
Input from section 

foremen. 

6 


Belt monitoring. .......... 


Belt fire monitor 


Parameters monitored. 
Size: 


Belt operation, belt 
sequencing, motor tem- 
perature and power, CO. 

40 

10 

2- and 4-conductor 

Digital 


equipment status. 
CO, fan temperature and 
pressure, degasifica- 
tion pumps, fire sup- 
pression operation. 

15 to 20. 


Input per station.. 
Telemetry: 

Cables 


NA 


1 6 maximum . 






Format 


Voice and digital 


conductor, shielded. 
Digital. 
Line printer, CRT. 

2-level. 


Data display 


Line printer, CRT's above 

and below ground. 
No 


Line printer, light panel. 
Yes 


Year installed 


1978 


1978 


1978. 


Comments ....•.•.■...• 


Production reports, man- 
power planning. 


Planned expansion — air 
velocity, CH4 monitoring. 


Allowed 75,326 




variance. 




D 


E 


F 


Function. ............ 


Belt fire monitor. •••■■•■• 


Environmental, fan opera- 
tion, roof falls. 

CO, CH4 , air velocity, fan 
power and pressure, seis- 
mic activity, battery 
voltage. 

11 

4 


Fan operation, haulage, 
fire, environmental. 

Fan operation, haulage, 
heat-CO, CH4 . 

26. 

16 maximum. 


Parameters monitored. 

Size: 

Remote stations.... 

Input per station.. 
Telemetry: 

Cables 


CO 

13 

1 

4-conductor, shielded 

Digital 




Digital 


shielded. 
Digital. 
Line printer, CRT, 

light panel. 
Yes. 


Data display 




Line printer, plotter, 
CRT, light panel. 

Yes 

1974 


At station and aboveground 
1981 




1978. 


Comments. 


Allowed 75.326 variance, 
planned expansion — belt 
monitor, ventilation 
monitor. 


Result of cooperative 
agreement with Bureau of 
Mines. 


Planned expansion — fan 
temperature, vibra- 
tion, maintenance 
data. 




G 


H 


I 


Function. ■■...■...... 


Fan operation. ............ 


Environmental monitoring.. 

CH4 , CO, temperature, rel- 
ative humidity, airflow, 
differential pressure. 

12 

10 to 20 


Methane drainage. 
CH4. 

>4. 


Parameters monitored. 

Size: 

Remote stations. ... 


Fan operation, tempera- 
ture, pressure, 
vibration. 

10 


Input per station.. 
Telemetry: 

Cables 


8 


1. 


Radio link.. •..■•..••■...•• 


2— conductor. .............. 


4— conductor . 


Format ..•.....•..•• 


FM 


Digital. • 


Analog. 

Strip chart, meter. 

2-level. 


Data display 

Alarm. .•.......••.■.. 


Line printer, light panel. 
Yes 


Mine maps, CRT, teletype.. 
2-level 


Year installed. ...... 


1979 


1972 


1977. 




Had prototype haulage mon- 
itor, planned expansion — 
maintenance data. 


WVU experiment , Bureau of 
Mines sponsored. 


Controls drainage 
valves, result of 
Bureau of Mines 
Research. 





NA Not available. 



NOTE. — Alphabetic designators are those used for mine designations in text. 



115 



4.6.2 U.S. Underground Metal-Nonmetal 
Mines 

Mine J 

Although mine J is not a coal mine, 
its physical layout and mining technique 
are very coal-like. It is a very exten- 
sive, single-level, room and pillar mine 
that extracts about 5 million tons per 
year, working about 7 feet of a 9-foot 
seam that is 1,200 feet deep. The mine 
is classified gassy because of the pres- 
ence of oil shale above and below the 
seam. However, it is allowed use of the 
beltway for fresh air, since metal- 
nonmetal mines are governed by 30 CFR, 
Part 57, which is less restrictive than 
the sections governing coal. 

Up to 10 continuous miner sections 
and a longwall are operated. Underground 
equipment is electric, with a combination 
of rail and belt haulage. 

A very satisfactory 50-station haul- 
age monitoring system has been installed, 
and an additional system is on or- 
der. There is about 20 miles of belt 
conveyors. 

The system, which was built by Aqua- 
trol Corp., monitors belt functions such 
as belt slip, plug up, sequencing, align- 
ment, etc. 

Mine K 

Operator K operates a multilevel 
copper mine, and has an ambitious load- 
shedding program underway. The haulage 
is electric-powered track and belt. Face 
operation is with diesel-powered track- 
less vehicles. Sump pumps run up to 
5,000 hp. The utility bill is based on a 
15-min peak load factor, which is hoped 
to be reduced greatly by load shedding. 
The hardware and telemetry will be pur- 
chased from Harris Co. Enlargement to a 
mine-wide supervisory control and data 
acquisition system, including airflow, 
perhaps production monitoring data, etc. , 



is anticipated. An example with rapid 
payback is automatic measurement of air 
quality following a large explosive shot. 
At present, a rescue team is dispatched 
to certify the work areas. The result is 
a 3- or 4-hour mine-wide shutdown, which 
is, of course, expensive. 

4.6.3 Foreign Mines 

Canada 

As in many countries, mine monitor- 
ing in Canada has become more important 
in recent years. The Canadian Mining Re- 
search Laboratories (CANMET) has been 
working on carbon monoxide and methane 
monitoring in western Canada since 1976. 
One such monitoring system, which uses an 
electrochemical carbon monoxide analyzer, 
was installed in a Kaiser Resources mine 
to detect mine heating. Air samples were 
drawn through 1/2-inch OD polyethelene 
tubing from distances up to 7,000 feet 
(8^). In 1978, this system detected a 
significant rise in the carbon monoxide 
levels in one return, indicating the po- 
tential onset of heating. This early in- 
dication of heating was credited with 
allowing the mine to alleviate the situ- 
ation without interrupting the mining 
schedule. 

CANMET began working on methane mon- 
itoring in the coal mines in western Can- 
ada in 1977. This work centered around 
the use of remote methane analyzers, 
again by drawing air samples through 
polyethelene tubing. In one mine with a 
history of sudden gas bursts, the system 
successfully documented the sudden meth- 
ane liberations (8) . 

More recently, the Cape Breton De- 
velopment Corp. has installed a computer- 
based mine monitoring system in its coal 
mine in Nova Scotia (6^). The system, 
which monitors methane concentrations, 
air velocities, air pressures, fan vibra- 
tion, machine temperatures, and methane 
pump pressure, was supplied by Transmit- 
ton Ltd. 



116 



Poland 

As is true for much of Europe, the 
Polish coal mining regulations consider 
continuous methane monitoring in the 
return airways with automatic deener- 
gizing as an adequate alternative to 
individual monitoring on each piece 
of face equipment (as is done in the 
United States). Furthermore, the methane 
threshold values permitted by the regu- 
lations are also increased in mines where 
continuous monitoring and automatic de- 
energizing are used (16). As discussed 
earlier, such regulations tend to encour- 
age the use of remote mine monitoring 
systems . 

The Polish Research and Development 
Center for Mining Mechanization, Electro- 
technics and Automation Systems (EMAG) 
has been working on ventilation monitor- 
ing and control systems for Polish coal 
mines for a number of years. The methane 
monitoring systems currently consist of 
remote sensors, telemetering equipment, 
and a miniprocessor , located in a central 
station that receives and analyzes the 
data. The system has the capability of 
automatically switching off electric pow- 
er if the methane concentrations exceed 
the specified level. The system also 
maintains permanent records of warnings 
and alarms and provides summary reports 
on methane liberation cycles, etc. 



interest in methane monitoring and con- 
trol is not great. The Swedish mines do 
use diesels , however, and an effort is 
underway to monitor carbon monoxide, 
nitrogen dioxide, etc. In addition, some 
work has been done on the development of 
automatic air regulation. Because meth- 
ane liberation is not a problem in Swed- 
ish mines, mine air regulation, which 
would be based on the number of diesels 
operating and on their location, is cur- 
rently being studied (19) . 

U.S.S.R. 

Automatic monitoring and control of 
mine ventilation systems has been studied 
in the U.S.S.R. for a number of years. 
This work has resulted in the development 
of an automatic ventilation monitoring 
and control system called ATMOS. This 
computer-based system is reportedly (14) 
able to monitor ventilation parameters 
(such as methane concentration, airflow, 
etc.), calculate the required airflows, 
and provide the system operator with in- 
formation on the appropriate fan and reg- 
ulator settings. Ventilation corrections 
are made on a weekly basis. 

The system has been operationally 
tested in two mines, and the Ministry of 
Coal Mining is currently in the process 
of commercial development of the ATMOS 
system. 



For fire monitoring, EMAG has also 
developed a carbon monoxide monitoring 
system based on a catalytic sensor and an 
ionization fume sensor. The system has 
been tested, experimentally, in Polish 
coal mines. 

EMAG has also worked on developing 
automatically controlled air regulators 
for coal mines. The regulator, which 
has pneumatically or hydraulically actu- 
ated vanes , is intended to become part of 
an overall mine monitoring and control 
system. 

Sweden 

Since most of the mining in Swe- 
den is metal mining in nongassy mines. 



United Kingdom 

The development and use of remote 
mine monitoring systems is probably more 
advanced in the United Kingdom than in 
any other country, apparently for two 
reasons. 

The first is that British mining 
regulations tend to encourage the use of 
such systems. For example, in contrast 
with U.S. regulations requiring that 
methane monitoring devices be installed 
on each piece of face equipment that can 
deenergize the equipment once the estab- 
lished methane threshold is exceeded, 
British regulations permit monitoring of 
the return airways for methane if the 
data are transmitted to a central control 



117 



station that can remotely deenergize the 
affected equipment. 

The second, and probably the more 
important reason, is that the British 
coal industry is nationalized and is sup- 
ported by a centralized research organi- 
zation (the National Coal Board's Mining 
Research and Development Establishment — 
MRDE). This arrangement has greatly fa- 
cilitated the development, testing, and 
implementation of mine monitoring systems 
in the United Kingdom. 

In the 1970' s, the MRDE focused its 
attention on developing a universal moni- 
toring and control system that could be 
used throughout the coal mining industry. 
The system, called MINOS (for Mine Opera- 
ting System) , is based on a common core 
of equipment that consists of a control 
console, central computer(s), and periph- 
erals. The application software is also 
the result of MRDE development. The mon- 
itoring systems are supplied to the mine 
by several independent companies that 
are free to market a variety of trans- 
ducers, data transmission equipment, and 
accessories. The concept of a universal 
computer-based operating system has per- 
mitted the MRDE to achieve certain econo- 
mies in the development of the system and 
tends to reduce interface problems. 

The applications of the MINOS moni- 
toring/control systems can be divided in- 
to the following six basic categories: 

1. Ventilation monitoring. 

2. Coal face monitoring. 

3. Coal clearance monitoring. 

4. Coal conveying monitoring. 

5. Fired plant monitoring. 

6. Preparation plant monitoring. 

Ventilation monitoring in the United 
Kingdom is currently being accomplished 
via tube bundle air sampling as well as 
telemetering of data from electromechan- 
ical transducers. In the former, air 



samples are drawn through tubes and ana- 
lyzed by using a gas analyzer located in 
a surface lab facility. In the latter, 
the output of the transducers is fed to 
outstations that encode and transmit the 
data to the surface via a communications 
cable. In either case, the environmental 
data (methane, oxygen, carbon monoxide 
levels, airflow, differential pressure, 
etc.) are analyzed, stored, and displayed 
on video monitors located on the central 
control panel. The displays and hard- 
copy reports can consist of warnings, 
alarms , actual values , or graphs of long- 
term trends (9). As of 1980, there were 
approximately two such systems either in 
operation or scheduled for installation 
(3). 

Development of a system for moni- 
toring face equipment performance began 
in 1977. One version of the system, 
called FIDO (Face Information Digested 
On-Line) was installed and tested in four 
collieries by 1980. The National Coal 
Board plans to install the system in an 
additional 24 collieries that have ap- 
proximately 100 active coal faces (20). 
Although the system originally monitored 
only face equipment operations, the NCB 
plans to expand the system to provide 
data on such parameters as roof height, 
pick force, and equipment orienta- 
tion, and eventually to permit automatic 
control of such equipment as longwall 
shearers. 

Monitoring and control of under- 
ground coal conveying systems is rela- 
tively advanced in the United Kingdom, 
with the first system in operation in 
1972. The systems provide stop-start 
logic sequencing in addition to sensing 
of such parameters as bearing tempera- 
tures, blocked chutes, motor operation, 
etc. As of 1980, there were approximate- 
ly 30 such systems in operation in the 
United Kingdom ( 20 ) . 

Monitoring and control of coal 
preparation plant operations (such as 
conveying, reagent mixing, etc.) is a 
relatively new application for the MINOS 
system. The system began development 
testing in the Lea Hall colliery in 1978. 



118 



A decision on expanding the use of the 
system will depend on the results of this 
inplant demonstration. 

It should be pointed out that mine 
monitoring systems based on the MINOS 
concept are currently being manufactured 
in the United States. 

Germany 

In recent years, remote monitoring 
of methane and carbon monoxide has re- 
ceived increased attention in German coal 
mines. One reason is that the German 
Federal Regulations on mine health and 
safety make such systems desirable and, 
in some cases, necessary. For example, 
the German regulations permit higher 
methane threshold values if constant mon- 
itoring is carried out by permanently in- 
stalled recording instruments that can 
telemeter the data to a remote control 
center that can automatically deenergize 
the electrical face equipment (16) . An- 
other example is the German requirement 
for automatic recording of carbon monox- 
ide levels along all belt entries. 

It has been estimated that as many 
as 1,400 methane and 1,200 carbon mon- 
oxide measuring devices were in use in 
the Ruhr district in 1978. Most of these 
provided remote transmission of the mea- 
surement data to a central control sta- 
tion. In addition to the methane and 
carbon monoxide sensors, some 500 fixed- 
point air velocity sensors were also es- 
timated to be operating in Ruhr district 
mines as of 1978. 

In addition, investigations have 
been conducted in Germany into the use of 
minicomputers as well as microcomputers 
to receive and process the data from the 
remote sensors. The primary purposes are 
(1) the reduction of false alarms through 
trend identification and signature 



matching and (2) the manipulation, pres- 
entation, and storage of large amounts of 
monitoring data. 

South Africa 

Remote automatic detection of un- 
derground mine fires is a major concern 
for South African mining companies. This 
is particularly true for the deep level 
gold mines because of the large amount 
of timber required for roof support in 
these mines. Instances of spontaneous 
combustion in South African coal mines 
have also been reported in recent years. 
For example, between 1968 and 1973 more 
than 23 mine fires occurred in one South 
African mining district. Approximately 
65% of these were attributed to spontane- 
ous combustion (11). 

Two basic approaches have been used 
in remote monitoring for underground mine 
fires. In coal mines, an infrared gas 
analyzer drawing air samples through 
polyethelene tubes has been used to sense 
carbon monoxide levels ( 11 ) , and a com- 
bination of infrared gas analyzers and 
ionization chamber detectors has been 
used in the underground gold mines (23). 
In the latter case, the electrical sig- 
nals from the transducers were tele- 
metered to a control room located above 
ground. In the control room, data (car- 
bon monoxide level for the gas analyzer 
and ionization level for the combustion 
and particle detector) are recorded in 
analog form on continuous logs. In addi- 
tion, the system has the capacity to 
initiate alarms if specified levels are 
exceeded. 

Although some difficulties were en- 
countered with the ancillary equipment 
(such as recorders) in the tube system 
and dirt and condensation in the telem- 
etry system, both have proven effective 
in detecting mine fires. 



REFERENCES 



119 



1. Aldridge, M. D. , and R. S. Nutter, 
An Experimental Ventilation Control Sys- 
tem. Proc. 2d Internat. Mine Ventila- 
tion Cong., Nov. 4-8, 1979, Reno, Nev. , 
pp. 230-238. 

2. Aldridge, M. D. , N. S. Smith, 
R. E. Swartwout, and D. T. Worrell. Con- 
clusions From the WVU Monitoring Experi- 
ments. Proc. 2d WVU Conf. on Coal Mine 
Electrotechnology , June 12-14, 1974, Mor- 
gantown, W. Va. , pp. 18-1 — 18-4. 

3. Barham, D. K. Progress in Coal- 
face Monitoring and Control in the 
United Kingdom. Proc. 5th WVU Conf. 
on Coal Mine Electrotechnology, July 30- 
31 — August 1, 1980, Morgantown, W. Va. , 
pp. 16-1 — 16-18. 

4. Bredeson, J. G, H. Hashemi, and 
K. Snedhar. Data Security for In-Mine 
Transmission. Final Report — Part 11. 
BuMines contract J0308024; for informa- 
tion contact R. W. Watson, Pittsburgh 
Research Center, Pittsburgh, Pa. 

5. Bredeson, J. G. , J. L. Kohler, and 
H. Singh. Data Security for In-Mine 
Transmission. Final Report — Part 1. Bu- 
Mines OFR 76-81, February 1981, HI pp.; 
NTIS PB 81-221998. 



6. Brezovec, 
Mine Conditions. 
August 1981, pp. 



D. Computer Monitors 
Coal Age, v. 86, No. 8, 
56-60. 



7. Cervik, 
ane Drainage 



J. Experience With Meth- 
From Horizontal Bore- 
holes. Proc. 2d Internat. Mine Ventila- 
tion Cong., Nov, 4-8, 1979, Reno, Nev., 
pp. 257-264. 

8. Chakravorty, R. N. , and R. L. 
Woolf. Environmental Monitoring for 
Safety in Underground Coal Mining. CIM 
Bull., V. 72, No. 801, January 1979, 
pp. 100-107. 

9. Corbett, A. W. The Use of Compu- 
ters for the Continuous Monitoring of the 
Environment at Brodsworth Colliery. Min. 
Eng. (London), v. 138, No. 210, March 
1979, pp. 641-650. 



10. Irani, M. C. , F. F. Kapsch, P. W. 
Jeran, and S. J. Pepperney. A Fail-Safe 
Control System for a Mine Methane Pipe- 
line. BuMines RI 8424, 1980, 11 pp. 

11. Joubert, F. E. , I. F. Buchan, and 
J. D. R, Beukes. Report on Carbon Monox- 
ide Monitoring System for the Early De- 
tection of Incipient Fires in Mines. J. 
Mine Ventilation Soc. South Africa, v. 
29, No. 4, April 1976, pp. 74-80. 

12. Kacmar, R. M. Reliability of 
Computerized Mine-Monitoring Systems. 
BuMines IC 8882, 1982, 12 pp. 

13. Kohler, J. L. An Evaluation of 
the Air Velocity Sensing Unit in the Bu- 
reau of Mines Remote Monitoring Sys- 
tem. Ongoing BuMines contract J0308027; 
for information contact E. D. Thimons , 
Pittsburgh Research Center, Pittsburgh, 
Pa. 

14. Kot, V. I., and E. F. Karpov. 
Coal Mine Ventilation - Monitoring and 
Control. Proc. 5th WVU Conf. on 
Coal Mine Electrotechnology, July 30- 
31~Aug. 1, 1980, Morgantown, W. Va. , 
pp. 15-1 — 15-14. 

15. Miller, E. J., P. M. Turcic, and 
J. L. Banfield. Equivalency Tests for 
Fire Detection Systems for Underground 
Coal Mines Using Low Level Carbon Monox- 
ide Monitors. Proc. 2d Internat. Mine 
Ventilation Cong., Nov. 4-8, 1979, Reno, 
Nev. , pp. 236-246. 

16. North American Mining Consultants 
Inc. Single Entry Longwall Study. U.S. 
Dept. Energy contract ET-77-C-01-9052, 
October 1979; available from U.S. Depart- 
ment of Energy, Germantown, Md. 

17. Nutter, R. S. Hazard Evalua- 
tion Methodology for Computer Controlled 
Mine Monitoring/Control System. Pres. 
at Western Electro Technology Conf., 
Reno, Nev., September 1981; available 
from R. S. Nutter, West Virginia Univer- 
sity, Morgantown, W. Va. 



120 



18. Prosser, L. J. , Jr. , G. L. Fin- 
finger, and J. Cervik. Methane Drain- 
age Study Using an Underground Pipeline, 
Marianna Mine 58. BuMines Rl 8577, 1981, 
21 pp. 

19. Rustan, A. Review of Develop- 
ments in Monitoring and Control of Mine 
Ventilation Systems. Proc. 2d Internat. 
Mine Ventilation Cong., Nov. 4-8, 1979, 
Reno, Nev. , pp. 223-229. 

20. Tregelles, P. G. Automation in 
UK Mines: Achievements, Goals & Prob- 
lems. Coal Min. and Processing, v. 17, 
No. 9, September 1980, pp. 110-122. 



Instrumentation. BuMines OFR 1-82, June 
1981, 137 pp.; NTIS PB 82-146325. 

22. Tongue, D. W. , D. D. Schuster, R. 
Neidbala, and D. M. Bondurant. Design 
and Recommended Specifications for a Safe 
Methane Gas Piping System. BuMines OFR 
109-76, July 1976, 97 pp.; NTIS PB 259 
340. 

23. Van der Walt, N. T. , B. J. Bout, 
Q. S. Anderson, and T. J. Newington. 
All-Analogue Fire Detection System for 
South African Gold Mines. J. Mine Venti- 
lation Soc. South Africa, v. 33, No. 1, 
January 1980, pp. 2-13. 



21. Trelewicz, K. Environmental Test 
Criteria for the Acceptability of Mine 



CHAPTER 5.— COMMUNICATION SYSTEM DESIGN AND IMPROVEMENT 



121 



5.1 Introduction 

This chapter analyses the parameters 
influencing initial design of communica- 
tion systems, for new mines and upgrading 
existing systems. 

Paragraph 5.2 outlines those varia- 
bles that must be taken into account dur- 
ing the design stages of a new wired 
phone system. Recommended features, gen- 
eral requirements, and how they can be 
implemented are treated in this section. 

Paragraph 5.3 describes ways of 
improving or extending the range of trol- 
ley carrier phone systems and pager phone 
systems already installed in the mine. 

5.2 New Phone System Design 

The task of designing an adequate 
communication, control, and monitoring 
system for an underground mine must be 
addressed on a system basis. In addition 
to insuring that effective voice communi- 
cation is established, any new system 
should take into account present and 
future requirements of remote control and 
monitoring functions. Chapter 4 illus- 
trated the drastic savings in response 
time that can be realized when remote 
control and monitoring are integrated 
into the overall communication system. 
The importance of including control and 
monitoring in the overall design plan for 
any system cannot be overemphasized. 

Because each mine is unique, and 
thus usually has its own special operat- 
ing characteristics and communication 
requirements, there is no such thing as 
"the one best system" to meet the 
requirements of all mines. The optimum 
communication, control, and monitoring 
system for a mine must be one that has 
been tailored to meet the special 
requirements of that particular mine. 
Factors that must be considered during 
system design include — 

a. Type of mine and mining methods 
(low- or high-seam coal, deep hardrock 



mine, stope caving, longwall, room and 
pillar, etc.). 

b. Maximum number of working 
sections. 

c. Expected mine growth rate and 
eventual maximum size. 

d. Haulage methods (tracked trol- 
ley, diesel, belt, etc.). 

e. Underground power distribution 
system (dc, ac, or both). 

f. Features desired (two-way radio 
paging, private line capability for emer- 
gency use, etc.). 

g. Redundant or backup systems 
for use during outages of the normal 
system. 

Although no two mines are alike, the 
following items have been established as 
the main characteristics desired for any 
underground communication system: 

1. Multiple Communication Paths to 
Outside — the objective here is to give 
all telephones a second method of commu- 
nicating with the surface. 

2. Audible Emergency Signaling — the 
communication system provides the main 
means of alerting miners during emergen- 
cies. The system should include means to 
broadcast distinct audible signals for 
emergency signaling. Initiation of these 
signals should probably be controlled 
from a central outside point, such as a 
surface control room. 

3. Emergency Override — provisions 
should be included to permit any con- 
versation to be overridden with emergency 
communication. 

4. Selective Area Page — as mines 
grow larger it is apparent that the 
entire telephone system paging mode need 
not be activated each time a call is 
initiated. When the general area of a 



122 



person to be paged is known, only the 
pagers in that area would be activated. 

5. Simultaneous Conversation Capa- 
bility — although the ultimate for this 
characteristic would be a private line 
for each telephone, this channel capacity 
may not be necessary in some mines. In 
general, each working section does not 
produce much communication activity. 
Haulage and maintenance activities domi- 
nate telephone use. Since these activi- 
ties tend to originate on the basis of 
mine "areas," it appears that providing 
different areas of the mine with a sepa- 
rate communication circuit could meet the 
simultaneous conversation need and main- 
tain circuit simplicity. 

6. Manual or Automatic Connection 
Between Subsystems — provisions must be 
made for connecting telephones within the 
telephone system, and provisions should 
be made for connecting the telephone sys- 
tem into the other communication systems 
used at the mine. 

7. Remote Signaling — the design of 
the telephone equipment and circuits 
should be compatible with frequency divi- 
sion multiplexed equipment so frequencies 
above 3,000 Hz can be used for control 
and monitoring applications. 

5.2.1 Wired Phone Systems 

The options open to a designer dur- 
ing planning stages for a hard-wired 
phone system include 

Single-pair phone system 

Multipair phone system 

Multiplexed phone system 

5.2.1a Single-Pair Systems 

Many different types of wire can be 
used for single-pair (party-line) commun- 
ication systems (table 5-1). Smaller 
gage wire may be satisfactory if the num- 
ber of telephones in the system is small 
and the distance between them is short. 
However, for most applications, a larger 



gage wire is chosen to improve the ten- 
sile strength of the wire, as well as to 
reduce the overall resistance of the run. 

TABLE 5-1. - Single-pair cable 



Description 


Wire 
gage, 
AWG 


Loop resist- 
ance , ohms 
per mile 


Plastic-insulated 
nonjacketed build- 
ing wire. .......... 


18 

18 
16 
14 

19 

18 


67 


Type SO, neoprene- 
jacketed portable 
cable 

Buried distribution 
wire. .............. 


67 
42 
27 

83 


Plastic drop wire 
(copper-clad steel) 


223 



An inexpensive wire used for inter- 
connecting mine phones is vinyl-plastic- 
coated, 18-gage, two-wire, twisted-pair 
building wire. Unjacketed wire of this 
type provides little environmental pro- 
tection for the copper conductors ; there- 
fore it must be located out of the way of 
the mining equipment and carefully sus- 
pended to avoid moisture penetration. 

The 14-gage neoprene-jacketed type 
(see fig. 5-1) is recommended for most 
underground applications. The greater 
mechanical strength, reduced loop resist- 
ance, and superior moisture resistance of 
this cable makes it ideal for communica- 
tion applications. 

The best method of getting a feel 
for the design considerations of a 
single-pair system is to design a system 
for a representative moderate-sized mine. 
An example of such a mine is shown in 
figure 5-2. This mine has the following 
characteristics: 




FIGURE 5-1, • Single-pair type SO neoprene cable. 



123 



^—SINGLE PAfR UNDERGROUND 

SINGLE PAIR FOR LOOP BACK ABOVE GROUND 

@ PERMANENT TELEPHONES IN MAIN HAULAGEWA 
O SEMI-PERMANENT TELEPHONES AT BUTT ENTRM 
® FREQUENTLY MOVED TELEPHONES AT WORKING 




FIGURE 5-2, - Single-pair installation in typical mine. 

Less than 2 years old 

6 square miles in total area 

3.5 miles of main haulageway 

0.8-mile-long average submain 

Average panel size of 800 feet by 
2,100 feet 

Average working section size of 
300 feet by 400 feet 

5 working sections per shift 

A maximum of 6 active working 
sections 

17 fixed mine pager phones presently 
installed 

The fixed-telephone, single-pair 
communication system shown in figure 5-2 
complies with the Federal Coal Mine 



Health and Safety Act of 1969, in that it 
provides two-way communication between 
the surface and each working section. 
Additional phones were installed at the 
intersections of the main haulageway and 
the submains , and at the intersections of 
the submains and the butt entries to all 
active sections. 

Based on the physical characteris- 
tics of the mine, the total length of 
single-pair cable required can be calcu- 
lated for this stage of development as 
follows: 

Miles 

1 main haulageway 3.5 

3 submains (0.8 mile 

each) 2.4 

6 active sections 

(3,000 feet per section). 3.4 

Total 9.3 

The 3.4 miles of section cable as- 
sumes the reuse of the cable as the work- 
ing sections move from one panel to 
another. At this stage in the mine's 
development, 15 panels have been driven 
or are being driven which would have 
required 8.5 miles of section cable if 
reusing it had not been assumed. There- 
fore, the total cable miles needed are 

9.3 if section cable reused 

14.4 if section cable not reused 

The least expensive wire for the 
above application is plastic-insulated, 
nonjacketed 18 AWG building wire. How- 
ever, the high loop resistance (67 ohms 
per mile) of the 18-gage wire will make 
future expansion impractical; therefore 
we should consider a larger gage wire. 

A more suitable cable due to its low 
loop resistance is 14 AWG, type SO, neo- 
prene wire. The 14 AWG neoprene cable 
uses annealed copper conductors so that 
it can withstand severe mechanical abuse. 
(The cable is designed for use as power 
supply cable on portable equipment.) If 
the 3,000 feet of 14 AWG neoprene wire 
used for each active section is mounted 



124 



on a reel and travels with the working 
section phone into the panel, then we can 
plan on reusing this wire when developing 
future panels. The cost of expanding to 
6 submains and 60 panels would involve 
only the additional wire for 3 submains, 
assuming we can reuse the section wire. 



replacing the single-pair cable in the 
main haulageway and the submains with 
multipair cable. In a new mine, it would 
mean calculating the maximum channel 
requirements expected during the life of 
the mine and specifying the proper multi- 
pair distribution system. 



The economic importance of reusing 
section wire can be elaborated on by the 
following calculations for 54 lengths of 
additional section wire needed to reach 
the 6-submain development stage if the 
section wire is not reused. Each length 
is 3,000 feet, or 0.57 mile. 

54 lengths x 0.57 mile per length 
= 30 miles of additional cable 

The cost of this additional cable 
can be a significant part of the total 
cost of the entire single-pair system. 
Although material costs are greatly 
reduced if section wire is reused, some 
additional labor costs are involved in 
the removal of cable once a panel has 
been completed. 

Another alternative that can be 
employed is to use high-quality 14 AWG 
wire for the main and submains , and then 
use a less expensive lighter gage wire 
for the panels and not reuse this wire. 
A low-cost 18-gage building wire may be 
acceptable as section wire, because its 
high resistance is not a problem for the 
short length involved. 

5.2.1b Multipair Systems 

A single-pair cable system restricts 
the mine communication system to a 
single-channel multiparty configuration. 
Introducing multipair cable into the mine 
communication system allows one to expand 
the number of channels to whatever is 
necessary for efficient voice traffic. 
In an existing mine, this would mean 



The hardware for a multipair system 
is of proven reliability and has stood 
the test of time. All of these materials 
have been used for aerial distribution 
systems in the telephone industry and 
were refined over the years to survive in 
any part of the world with a minimum of 
preventive maintenance. Because it was 
designed to be installed and maintained 
by linemen working in all kinds of 
weather while standing on ladders, on 
aerial platforms, or in manholes, multi- 
pair equipment can be handled by elec- 
tricians in the underground environment. 
The only new skill that mine personnel 
may have to learn is the splicing of 
small-diameter wires. However, crimp- 
type splice connectors are available to 
simplify the splicing of multipair 
cables. 

Table 5-2 shows the major character- 
istics of multipair cable available from 
telecommunication cable manufacturers. 
Figure-8 cable is recommended for the 
mine application because the messenger 
wire adds considerable tensile strength 
to the cable, and the installation is 
similar to that of trolley wire. 

The previous section described a 
single-pair cable system using a repre- 
sentative moderate-size fictitious mine. 
The same mine will be used to analyze a 
multipair cable system (fig. 5-3). 

Using a cable distribution and load- 
ing plan that will allow the servicing of 
no more than two sections per twisted 
pair, a minimum of three pairs is 



TABLE 5-2. - Range of multipair cables commercially available 



Number of pairs 3-400 

Messenger size (diameter) inch.. 0.134-0.250 

Conductor size AWG. . 26-19 

Conductor dc resistance at 68° F...ohms per mile.. 43-220 



125 




FIGURE 5»3, - Multipair installation in typical mine, 

required to handle the six working sec- 
tions. The main haulageway phones con- 
nected across a single party line require 
an additional pair for a total of four 
pairs, each of which extends back to a 
centralized location such as the dis- 
patcher's office. A six-pair cable 
placed in the main haulageway will accom- 
modate the above required pairs while 
leaving an extra two pairs for future 
expansion. Three-pair cable may be ap- 
propriate for the submalns because no 
more than four sections will be active 
per submain at any one time. A single- 
pair cable can be used between the panel 
entry phone, located in the submain, and 
the section phone, which must move with 
the section crew. 

Due to the 3.5-mile length of the 
main haulageway and assuming that a maxi- 
mum of seven phones will be connected 
in parallel across one pair, a 19-gage 



six-pair cable has been selected for the 
haulageway. The submains with only two 
phones per pair and run lengths of less 
than 1 mile can use 22-gage wire. A 
splice case at every third section entry 
should be sufficient in this application 
and will reduce labor costs. 

The section cable can be a single 
pair but must be strong enough to with- 
stand the wear caused by the almost con- 
stant phone relocating required in the 
working section. A 3,000-foot reel of 
wire that travels with the section phone 
would reach any location in an 800- by 
2,100-foot panel. Plastic drop wire has 
been chosen for the section cable. This 
wire is made up of two 18 AWG copper- 
covered steel wires laid in parallel and 
coated with a black flame-resistant poly- 
vinyl chloride insulation. The high 
strength of this cable allows for long 
spans which make for quick temporary in- 
stallations and also reuse of the cable. 
A stainless steel drop wire clamp can be 
hooked to roof bolts or nailed to timbers 
for support. 

In cost comparisons between single- 
pair and multipair systems (2^),^ the wir- 
ing costs for multipair installations 
were less expensive because the smaller 
gage wire allowed in the multipair cable, 
due to fewer phones placed in parallel 
per pairs, kept the per-mile cost of 
multipair cable competitive with that of 
the larger gage single-pair cable. 

Two questions worthy of considera- 
tion at this point are. How well does a 
multipair communication system meet the 
needs of the mine user? and What 
improvements can be incorporated into a 
multipair system that are not possible 
with the present day single-pair mine 
telephone system? 

Advantages 

More Channels . — Using multipair ca- 
ble, a system can be designed with as 

^Underlined numbers in parentheses re- 
fer to items in the bibliography at the 
end of this chapter. 



126 



many channels as are deemed necessary 
for the particular application, the only 
limits being cost and complexity. 

Private Channels. — Individual pairs 
can be assigned to each working section, 
thereby producing a private channel 
between the section and the mine commu- 
nications center. 

Zone Paging. — The communication cen- 
ter can page over an individual pair 
so that only the section of the mine 
concerned with the transmission need be 
disturbed. This would eliminate the 
present situation of requiring miners in 
all sections to listen to all pages. 

Direct Dialing. — Pairs can be dedi- 
cated to connect underground dial phones 
directly to the company's private auto- 
matic branch exchange (PABX) or directly 
to a central office through an approved 
interface. This would allow key loca- 
tions in the mine to dial each other, 
place outgoing calls, or receive incoming 
calls via the local exchange without 
relaying messages through the communica- 
tion center. Provisions for preventing 
abuse of the latter two features could 
also be included. 

Remote Monitoring. — Extra pairs in 
the cable may be used for monitoring the 
mine environment and/or equipment. 

Disadvantages 

Increased Operating Costs. — A multi- 
pair system incorporating all of the 
above advantages will cost more than a 
single-pair system, even though the 
multipair cable may cost less than the 
single-pair cable. This is due to the 
additional cost of a central switching 
equipment required for multipair systems. 
For a particular application, the 
increased efficiency and other benefits 
must be weighed against the added in- 
stallation and maintenance costs in order 
to establish its true worth. 

Training Costs. — The maintenance per- 
sonnel assigned to install and main- 
tain this equipment will have to be 



trained to use the different splicing 
techniques required and to troubleshoot 
this somewhat more complex system. 

5.2.1c Multiplexed Phone Systems 

Multiplex telephone systems achieve 
their private channel capability via 
electronic means on a single cable. 
Multiplexing can be via time division 
multiplexing (TDM) or frequency division 
multiplexing (FDM). Although TDM systems 
have been developed and provide certain 
advantages, a multitude of disadvantages 
tend to make this type of multiplexing 
unattractive for mine telephone systems. 

FDM systems have been developed and 
tested in underground mines with consid- 
erable success. These systems can be 
divided into ones that require a central 
switching station for system control and 
those that do not. In a central switch- 
ing system, most if not all of the system 
intelligence resides in the central unit 
which assigns frequencies, provides power 
for the phones, and generates ringing and 
busy signals. These systems are gener- 
ally permitted only in nongassy mines. A 
serious disadvantage of such a system is 
that a failure in the central unit can 
render the entire system inoperative. 

A system that does not rely on a 
central switching unit has been developed 
by the Bureau of Mines. The system is 
based upon microprocessor control, where 
intelligence is resident in each tele- 
phone. Eight-channel voice or data com- 
munications is possible. The system uti- 
lizes FDM at medium frequencies (340-650 
kHz) and is designed for a 10-mile cable 
plant. A failure of any one phone nor- 
mally affects the multiplex feature of 
that phone only. Each phone also in- 
cludes a resident pager phone capability 
such that even a total failure of the 
microprocessor intelligence will not 
normally inhibit a user from making a 
call. This feature is essential in any 
modern telephone system for underground 
mines. Supervisory feedback and a vis- 
ual message-leaving capability (as is 
required in several States) are also 
included. 



127 



5.2.2 Cable Selection 

Telephone transmission is made over 
wires which represent a considerable 
fraction of the cost of any telephone 
system. As an example, figure 5-4 shows 
three broad categories of equipment in 
which telephone companies invest. The 
"transmission" category not only repre- 
sents wires, but also includes multiplex 
systems, microwave systems, and other 
wire substitutes. Since transmission 
equipment accounts for about half of the 
total investment, telephone companies put 
considerable effort into planning the 
layout and the growth of their transmis- 
sion facilities. Cable costs account for 
even a greater percent of the expense in- 
volved in an underground communication 
system. Therefore, mine planners should 
also carefully plan the network and re- 
vise the plan on a scheduled basis. 

The general environment in an under- 
ground mine imposes severe physical re- 
quirements on communication cable. Insu- 
lation is required to withstand exposure 
to moisture, abrasion, and rough han- 
dling; to afford protection against some 
level of accidental contact with higher 




FIGURE 5-4, - Telephone company investments. 



voltages; and to not support combustion 
in case of fire. 

Twisted-pair construction is advised 
to reduce the effects of induced noise or 
interference. The 14 AWG solid-conductor 
twisted pair, with suitable insulation 
dielectric and outer protective jacket is 
very rugged, and will withstand the rough 
handling and stress imposed by abrasion 
against timbers or falling debris. For 
the smaller diameter wires, such as 
19 AWG, a figure-8 cable is recommended. 
In this construction, a steel "messenger" 
or support wire is added to the twisted- 
pair bundle, so that the overall cross 
section resembles a figure 8. The -steel 
messenger cable provides additional 
strength and support so that minimum 
strain is applied to the signal-carrying 
twisted pair. 

Solid conductor is advised, rather 
than the more easily handled multistrand 
wire. The multistrand cable is subject 
to corrosion buildup on the surface of 
the individual conductor strands, which 
in time could reduce the conductivity of 
a splice or connection and become the 
source of added noise and reduced signal 
level. Conditions within an underground 
mine dictate the use of press-on or 
twist-on connectors as common practice to 
complete a splice. Such practices are 
not compatible with the use of multi- 
strand wire. 

The choice of wire size is deter- 
mined by the configuration of the tele- 
phone system and the type of phone in 
use. Major factors to consider in the 
choice of wire size are the total length 
of cable run, the number of phones in the 
circuit, the average distance between the 
phones, and the characteristics of the 
ringing or calling circuit in each phone. 

In pager phone systems, the paging 
relay circuit is one of the more critical 
parameters to consider in the choice of 
pager phone wire size. The normal audio 
signal imposed on the cable is about 1 to 
2 MW; this signal level is sufficient to 
operate a phone receiver at satisfactory 



128 



volume over several miles of cable as 
small as 19 AWG. The limiting condition 
is the ability to reliably operate the 
paging relays. In this regard, the cable 
impedance, or resistance per unit length, 
as it affects the available dc voltage at 
the paging relay, is more influential 
than audio loss. Calculation of the min- 
imum wire size that will insure reliable 
operation of all paging relays must take 
into account three major parameters: 
paging circuit impedance, battery volt- 
age, and wire losses. 

Some pager phones use electromechan- 
ical relays that have an impedance of 
about 2,500 ohms while other systems use 
electronic or semiconductor switching 
circuits that have an impedance of from 
8,000 to 50,000 ohms. The minimum dc 
voltage required to operate any of these 
relays is about 1.5 to 4 volts. To 
insure a safety margin, it is recommended 
that at least 5 volts dc be available at 
all telephone paging relays. It is eas- 
ier to obtain this minimum voltage with 
the higher impedance circuits. 

Available battery voltage is a func- 
tion of the condition of the battery and 
the load it must operate. In a 12-volt 
system, the battery is at the end of its 
useful life when the dc voltage under 
load condition approaches 8 volts. For a 
24-volt system, a battery is at the end 
of its useful life when the available dc 



voltage under load approaches 16 volts. 
There is no specific time at which the 
battery can be identified as not usable. 
However, it is generally agreed that the 
levels just stated are typical of the end 
of a battery's useful life and indicate 
that it should be replaced. 

In many pager phones, the internal 
circuit has been designed so that the 
total battery voltage is not available on 
the line for operation of paging relays. 
Circuitry in such phones can add a series 
dc resistance of from 10 to 100 ohms to 
limit the short-circuit drain to levels 
of operation that are intrinsically safe. 
A pager phone system can draw significant 
current from the battery in the "paging" 
phone. This causes an internal voltage 
drop which significantly reduces the 
effective voltage presented to the line. 
Estimates of this effect, for a variety 
of conditions, are shown in table 5-3. 

Wire loss per unit length is a func- 
tion of wire diameter and system configu- 
ration. These factors include total wire 
used, telephone spacing, number of 
phones, and input impedance. All of 
these factors must be considered together 
in view of the expected battery voltage 
at end of useful life (8 or 16 volts), 
the relay impedance (2,500 ohms or 
greater than 8,000 ohms), and the inter- 
nal voltage drop because of circuit 
losses. 



TABLE 5-3. - Effect of paging circuit impedance 
(Electromechanical relays, 2,500 ohms) 



Battery voltage. 


Limiting 


Available battery voltage 


dc volts 


resistance, 
ohms 


on the line, dc volts 




10-phone system 


20-phone system 


24-volt battery: 








24 (new) 


10 
100 


23.75 
19 


23.5 
18 


16 (near end of life). 


10 
100 


15.5 
13 


15.5 
12 


12-volt battery: 

12 (new) 


10 
100 


11.8 
10 


11.6 
8 


8 (near end of life).. 


10 
100 


7.8 
6.5 


7.6 
6 



129 



The simplest calculation is to as- 
sume a basic ladder configuration, where 
all phones are in parallel on the same 
single two-wire cable, strung the length 
of the installation (fig. 5-5). This ba- 
sic installation is the one most normally 
considered when calculations are made to 
determine minimum wire size. Tables 5-4 
and 5-5 indicate the minimum wire size 
for both electromechanical and electronic 
relays, with average phone spacing of 1/4 
and i/2 mile. 

In a 12-volt system, with electro- 
mechanical 2,500-ohm relays, only 12 
hones spaced 1/4 mile apart over 3 miles 
can be used with 19 AWG wire. However, 
20 phones can be used over a 5-mile run 
if 14 AWG wire is used. If electronic 
8,000-ohm relays were used, the 24-volt 
system could support 33 phones over 8 
miles of cable using 19 AWG wire. 

Tables 5-4 and 5-5 do not take into 
consideration line losses caused by poor 
splices, dampness, or defective phones. 
However, they do illustrate comparative 
conditions as a guide for system design 
and component selection. 

Consideration of a topography that 
involves a multiple-branch system may 
result in a design that can use a smaller 
diameter wire. Conditions in mines nor- 
mally degrade even the best of systems — 
moisture causes signal leakage; erratic 



m 

o 



m 

o 



fqf 





Rw = LIME IMPEDANCE 

Rp - PHONE INPUT IMPEDANCE 



FIGURE 5-5. - Basic ladder configuration. 

or incorrect branch connections and 
splices tend to reduce performance — so 
that using detailed calculations to de- 
termine marginally usable minimum wire 
size is not a recommended practice. It 
makes more sense to determine a minimum 
wire size for safe operating level and 
then use that size as a guide to select 
or recommend a wire that meets all the 
specifications. For multiple-branch con- 
figurations, the following rules of thumb 
can be used to estimate minimum wire size 
without extensive calculation: 



1. Determine 
configuration. 



present telephone 



TABLE 5-4. - 1/4-mile pager phone spacing 



System 



19 AWG 



14 AWG 



12-volt, 2,500-ohm relay 

24-volt, 2,500-ohm relay 

12-volt, 8,000-ohm relay 

24-volt, 8,000-ohm relay 



3 miles, 12 phones 

5 miles, 20 phones 

5 miles, 20 phones 

8 miles, 33 phones 



5 miles, 
9 miles. 



20 phones 
36 phones 



9 miles, 36 phones 
>9 miles, >36 phones 



TABLE 5-5. - 1/2-mile pager phone spacing 



System 



19 AWG 



14 AWG 



12-volt, 2,500-ohm relay 

24-volt, 2,500-ohm relay 

12-volt, 8,000-ohm relay 

24-volt, 8,000-ohm relay 



4.5 miles, 9 phones 
7 miles, 14 phones 

7.5 miles, 15 phones 
13 miles, 26 phones 



7.5 miles, 
13 miles. 



15 phones 
26 phones 



13 miles, 26 phones 
>18 miles, >36 phones 



130 



2. Estimate probable growth of the 
telephone configuration. 

3. Sketch the future telephone 
configuration. 

4. Examine the sketch to determine 
the longest combined path that takes into 
account a majority of the telephones. 

5. From table 5-2 or 5-3 determine 
the minimum wire size for the longest 
path needs. 

6. The added loads of the other 
branches will not greatly affect the 
determination of minimum wire size and 
can be ignored for such an estimate. 

The 21 pager phones shown in the top 
panel of figure 5-6, spaced an average of 
1/4 mile apart, are connected in a 
branching system, which can be repre- 
sented by the impedance diagram shown in 
the bottom panel. The longest path is E 




(A) y,-MlLE PAGER PHONE SPACING 




(Bl EQUIVALENT CIRCUIT 

FIGURE 5-6. - Branching ladder network. 



to D to J, which includes 10 phones over 
about 2.5 miles of cable. If we examine 
table 5-2, we find that with 2,500-ohm 
mechanical relays in a 12-volt system, 
19 AWG wire is adequate for the 
configuration. 

This type of rule-of -thumb estimate 
is adequate to identify approximate 
requirements for wire size, but it does 
not replace necessary detailed calcula- 
tions for a major installation with many 
branches. It must also be emphasized 
that calculation of minimum wire size 
identifies the bottom limit of a marginal 
condition and good engineering practice 
dictates some margin of reliability. The 
general manufacturers' recommendation of 
14 to 16 AWG twisted pair for systems us- 
ing 2,500-ohm electromechanical relays is 
sound, particularly for a 12-volt system. 

For systems using semiconductor pag- 
ing circuits (with impedances of 8,000 
ohms or greater), 19 AWG is usually ade- 
quate. This is particularly true for 24- 
volt systems, but also applies to most 
12-volt systems that have high-impedance 
switching circuits. 

In summary, the cable wire size de- 
pends on a series of factors that include 
the total number of telephones in an in- 
stallation, the total length of cable run 
(distance between the farthest phones), 
the configuration of branch lines, the 
available battery voltage, and the type 
of paging relay used. The preferred ca- 
ble, regardless of wire size, is a 
twisted pair of solid conductor wires, 
with individual insulation around each 
wire in the pair and an outer abrasion- 
resistant covering of waterproof, flame- 
retardant material. 

5.2.3 Summary 

The basic system choices that may be 
selected when choosing an underground 
wired phone system consist of — 

Single pair . — This is a party line 
system in which all phones are on the 
same channel. 



131 



Multipair. — A private line system 
with each phone or group of phones con- 
nected to the system center by its own 
individual wire pair. 

Multiplex. — A private line system 
using a single cable, with the audio to 
and from each phone multiplexed onto the 
common cable. 

In all of these systems, telephone 
transmission is made over wires which 
represents a considerable fraction of the 
cost of the entire system. Since trans- 
mission equipment accounts for about half 
of total investment, companies should put 
considerable effort into planning the 
layout and growth of their transmission 
facilities. 

In planning mine communication 
systems, the pairs or voice channels 
that will be needed in the future and 
the mobility of the telephones involved 
should be kept in mind. In addition, 
pairs that will be needed for purposes 
other than for telephones (telemetry, 
remote monitoring, etc.), which inci- 
dentally may exceed voice communica- 
tion needs, should also be taken into 



performance of a trolley carrier system, 
and the second treats telephone systems. 

5.3.1 Trolley Carrier Phone Systems 



account. 



5.3 Improving Existing (In-Place) 
Phone Systems 

The two types of communication sys- 
tems commonly used to date in underground 
mines are as follows: 

Carrier current radio system using 
the trolley line. 

Various types of telephone system. 

Because these systems have gained 
such widespread usage, methods for up- 
grading and improving presently installed 
systems are presented in the following 
sections. The first deals with improving 

' Approved and nonapproved equipment may 
not share the same cables; check with 
MSHA for details. 



WARNING 

Some of these procedures are un- 
dertaken with the trolley wire ener- 
gized; therefore, they are extremely 
hazardous. Extreme caution must be 
exercised to avoid potentially lethal 
shock. The fuses used in the test 
leads serve only to protect equipment 
and do not in any way reduce the 
shock hazard to personnel. Only per- 
sonnel thoroughly familiar with elec- 
trical work on trolley wires should 
conduct these procedures. The perma- 
nent connection of components should 
be done with power removed. Care 
should also be taken to insure that 
components and equipment are suitable 
for use in the desired application. 



The trolley carrier phones used for 
dispatch purposes in electrical rail 
haulage mines often show problems in pro- 
viding coverage over the entire haulage 
system. Direct communication between the 
dispatcher and vehicles in certain areas 
of the mine is often difficult or impos- 
sible. The major reason for these diffi- 
culties is the effects that loads placed 
across the trolley wire or rail have on 
transmission. 

Both theory and experiment show that 
the trolley wire-rail by itself is a rel- 
atively good transmission line for car- 
rier phone frequencies. In fact, on an 
unloaded trolley wire-rail transmission 
line, a distance of 35 miles could be ex- 
pected for communication range. Communi- 
cations over a real trolley wire-rail can 
never achieve this range because the many 
loads across the trolley wire-rail absorb 
and reflect carrier signal power. The 
list of these loads is long and includes 
rectifiers, personnel heaters, signal 
lights, vehicle motors, vehicle lights, 
and the carrier phone itself. It is 
probable that the net signal attenuation 



132 



rate for a trolley wire-rail with typical 
loads placed across it yields a useful 
range as low as 3.5 miles. The problem 
of obtaining good signal propagation is 
further aggravated by branches of the 
trolley wire where the signal splits in a 
totally uncontrollable way. Lack of 
proper signal termination at the ends of 
the trolley wire-rail further degrades 
signal propagation. The vehicles repre- 
sent moving loads on the transmission 
line and add a further complication to 
obtaining or predicting good signal pro- 
pagation. Also, advancing the mine face 
means that the transmission network 
changes with time, yielding more uncer- 
tainty to the quality of transmission. 

The seriousness of the bridging 
loads can be seen by reference to fig- 
ure 5-7 where the losses for typical 
loads are tabulated. Using this chart, 
one can make an estimate of the total 
signal loss by adding the individual 
losses (in decibels). 

In the past, whenever poor trolley 
carrier communications existed, attempts 
were made to remedy the problem using 
"Z-boxes," or signal couplers to the 
phone line. Z-boxes are not permissible, 
are usually not the best solution, and 
may actually introduce more problems then 
they solve. Mines are full of Z-boxes 
that have been disconnected and abandoned 



fin 




LOAD 


OHMS 




\ 


HEATEHS 


JO -100 




RECTIFIERS 


2-10 




VEHICLE MOTORS 


60-500 


50 


VEHICLE LIGHTS 


60-120 




CARRIER PHONES 


20-200 




MINE LIGHTS 


500-2000 


40 

30 
20 
10 


1 1 





because of poor performance. It is 
recommended that solutions other than 
Z-boxes be used to improve the perform- 
ance of trolley carrier phone systems. 

The most straightforward way of 
treating the trolley wire-rail to make it 
into a functional carrier signal trans- 
mission line is to physically remove from 
the trolley wire-rail all of the bridging 
loads that impede carrier signal propaga- 
tion. The steps in this process follow: 

1. Identify the bridging loads. 
List all the bridging loads across the 
trolley wire-rail. Consult figure 5-7 to 
estimate the seriousness of the impedi- 
ment to carrier signal propagation that 
each load represents. 

2. Determine which loads can be 
removed from the trolley wire-rail and be 
operated from mine ac power. 

For practical reasons, physical 
removal of bridging loads has severe lim- 
itations. Certain critical loads, in- 
cluding rectifiers, vehicles, lights, 
motors, and carrier phones themselves, 
cannot be removed from the trolley wire- 
rail. In some instances, none of the 
loads can be removed from the trolley 
wire-rail, and efforts to improve signal 
propagation must involve other methods. 

Studies conducted have revealed 
alternative ways of increasing the range 
and quality of existing trolley carrier 
phone systems. These methods include — 



Isolated 
frequency 



loads at 



the 



carrier 



BRIDGING LOAO lOHMSI 



FIGURE 5-7. " Signal loss versus bridging load. 



Using a dedicated line 

Using a remote transceiver 

5.3.1a Isolating Loads at the Carrier 
Frequency 

Figure 5-7 shows that as the bridg- 
ing resistance is increased, the signal 
loss decreases. The "isolating loads" 
method involves adding passive circuit 
elements (inductors and capacitors) in 



133 



series with the particular load to reduce 
the effects of the bridging load. The 
circuit elements do not affect dc equip- 
ment (motors, lights, etc.) being powered 
from the trolley wire, but they do, if 
properly chosen, add high impedance at 
the carrier frequency. Rectifiers, heat- 
ers, and vehicle lights are the bridging 
loads that most seriously degrade re- 
ceived signal levels and should be 
treated first to improve received signal 
levels. 

5.3. la. i Rectifiers 

There are three means of raising the 
effective carrier frequency impedance of 
a rectifier. The most practical method 
depends on where the rectifier is in- 
stalled. If it is located relatively far 
from the rail (beyond 40 feet), the feed 
wires represent sufficient inductance 
that can be resonated, thereby raising 
the effective impedance as seen by the 
trolley wire-rail (fig. 5-84). If the 
rectifier setback is short (less than 40 
feet) , two techniques can be used to 
raise the effective impedance: (1) A 
fixed high-current inductor can be added 
in series with the rectifier and that 
inductor can then be tuned to raise the 
effective impedance (fig. 5-8S); or 
(2) the inductance of the trolley wire- 
rail can be used to resonate short sec- 
tions of the trolley wire-rail near the 
bridging load to raise the effective 
bridging impedance (fig. 5-8C). The ways 
of applying each of these means are de- 
scribed below. 

a. Resonating the Feed Wire Inductance 

The following steps are required to 
tune the rectifier feed wires: 

1. Attach a 1,000-volt (some sys- 
tems may require even higher voltage com- 
ponents) , l-pF or larger, oil-filled 
capacitor directly across the plus and 
minus terminals inside the rectifier. 
(This capacitor serves to reject 
rectifier-generated interference in the 
carrier frequency band. ) 

2. At the far end of the feeder 
wires, as near to the trolley wire-rail 



FEED WIRE INDUCTANCE 



ADDED TUNING CAPACITOR 




A RESONATING THE FEEDWIRE INDUCTANCE 



ADDED TUNING CAPACITOR 




ADDED FIXED INDUCTANCE 



B. RESONATING AN ADDED FIXED INDUCTANCE 




ADDED 
TUNING 
CAPACITOR 



C RESONATING THE TROLLEY WIRE/RAIL INDUCTANCE 

FIGURE 5-8. - Ways of raising the impedance 
of a rectifier. 

as practical, install the temporary test 
set shown in figure 5-9. This test set 
comprises a decade capacitor, isolating 
and protection devices, and a tuned 
voltmeter. Usually two feed wires are 
run from the rectifier to this point. 
Only one need be treated. 

3. The dispatcher is called from a 
jeep parked nearby and asked to key on 
his transmitter for 20 seconds or so. 
The decade capacitor box is switched 
through its range of operation and left 
at the position of maximum signal, as 
indicated by the tuned voltmeter. (The 
decade box should have enough range 
to peak the voltmeter.) This value of 
signal should be larger than when the 
decade capacitor is at its off position. 
The two values — the voltage when the dec- 
ade capacitor is off and the maximum 



134 



TO VOLTMETER 




WARNING I 

THIS CAPACITOR 

S ESSENTIAL WITHOUT 

EDO. TROLLEY 
VOLTAGE IS APPLIED TO 
THE VOLTMETER 

OR MORE 







FIGURE 5=9. => Test configurations for tuning 
feeder wire. 




FIGURE 5-10. - Permanent installation of tun= 
ing elements for feeder wire. 



WARNING 

Some of these procedures are undertaken with the trolley wire energized; there- 
fore, they are extremely hazardous. Extreme caution must be exercised to avoid 
potentially lethal shock. The fuses used in the test leads serve only to protect 
equipment and do not in any way reduce the shock hazard to personnel. Only per- 
sonnel thoroughly familiar with electrical work on trolley wires should conduct 
these procedures. The permanent connection of components should be done with 
power removed. Care should also be taken to insure that components and equipment 
are suitable for use in the desired application. 



value — should be logged, preferably on a 
mine map. There should be an appreciable 
increase in voltage for this condition, 
at least 1 1/2 to 1, and in some in- 
stances up to 10 to 1. The value of the 
capacitance that produces the maximum 
voltage should be noted from the value 
indicated on the decade capacitor, and a 
suitable capacitor of that value should 
then be installed in a permanent fashion, 
as shown in figure 5-10. When this in- 
stallation has been made, a final check, 
using the tuned voltmeter, should be made 
to ascertain that the originally indi- 
cated increased voltage is obtained. 

For this procedure, it is important 
that the tuned voltmeter be tuned to the 
precise transmission frequency of the 



dispatcher. A preliminary test can 
easily ascertain that this condition has 
been met by sweeping the tuning dial of 
the tuned voltmeter through the region 
near the transmitted frequency and leav- 
ing it at the position where maximum 
response is indicated. 

b. Resonating an Added Fixed Inductance 

When the setback is short, an added 
inductor made of a coil of feeder wire 
may be used to provide a series induct- 
ance that can be tuned. Because feeder 
wire is expensive, a coil in the so- 
called Brooks form, which yields the max- 
imum inductance per length of wire, 
should be used. See Appendix A (Mine E) 
for an actual installation example. 



135 



The approximate form is shown in 
figure 5-11. A reasonable bending radius 
for the typical thousand-circular-mils 
cable used for such feeder wires is 2 
feet; therefore this dimension is approx- 
imately fixed. Four turns at this diam- 
eter yield an inductance of approximately 
25 )H, which is adequate for tuning most 
rectifiers. The coil should be installed 
in the room in which the rectifier is lo- 
cated and should be kept a few feet away 
from the coal to prevent added losses at 
the carrier frequency. The exact value 
of inductance is unknown, so the coil 
will have to be tuned in much the same 
manner as discussed previously for reso- 
nating the feeder wires. 

Figure 5-12 illustrates the test 
setup. The dispatcher is called and 
asked for a 20-second transmission. The 
decade capacitor is switched through its 
positions and left at the position that 
yields the maximum voltage. (The decade 
box should have enough range to peak the 
voltmeter.) The received voltage with 
the decade capacitor in the "off" posi- 
tion and the maximum voltage should be 




NOTES 

MATERIAL 2Xd LUMBER 
DIAGONAL BRACKING. OMITTED FOR 
EASE OF DRAWING, IS REQUIRED 



5S 



noted, preferably on a mine map. When 
the best capacitor value has been found 
in this manner, the test set is re- 
moved and a suitable capacitor of the 
value found during the test is perma- 
nently attached to the coil, as shown in 
figure 5-13. When completed, a last test 
is made to verify that the improved sig- 
nal reception is obtained. 

c. Resonating the Trolley Wire-Rail 
Inductance 

A method that can be applied if the 
rectifier setback is short, and it would 
be impractical to install a fixed induc- 
tor in series with the rectifier feed 
wires, is to tune the trolley wire-rail 



1000 V 

DECADE 

CAPACITOR 



WARNING 







CAPACITOR 

5 mtd, 1000 VOLTS, 

IS ESSENTIAL WITHOUT 

IT TROLLEY VOLTAGE IS 

APPLIED TO THE 

VOLTMETER 



TROLLEY WIRE 



OR GROUND 







r 


r 


c 

\ 

f-: 


'^g'^ ' 


B 


"3 
3^" 


1 






E 1 



e PIECES A, THRU Ag 
4 PIECESe@3' -0" 
4 PIECESC@3' - II" 
2 PIECES D®4' -2" 
4 PIECES E®4'-5" 



J ' ' V '. . t 



WARNING 

Some oflliese procedures arc uiidcrtaketi with ihe trolley 
wire energized; therefore, they are extremely hazardous, 
fc-xtreme caution must be exercised to avoid ])otential!y 
lethal shock. The fuses used in the test leads serve only 
to jirotect eqiiipinent and do not in any way reduce the 
shock, hazard to personnel. Only personnel tliorouj^hly 
familiar witli electrical work on IroUev wires should con- 
duct these |)rocediires. The permanent connection ot 
components should be <lone with power removed. Care 
should also be taken to insme tlial components and equi])- 
meiit are suitable for use in the desired application. 



FIGURE 5-11.- Coil form. 



FIGURE 5-12. - Test setup for tuning fixed 
inductance. 



136 



CAPACITOR 
MOUNTED IN 
FUSE HOLDER 
IN UTILITY BOX 




a mine map. Remove the test fixture, and 
install permanently a suitable capacitor 
of the indicated value. Request another 
transmission to verify that the signal 
improvement observed during the test was 
maintained after permanent installation. 

2. Repeat the same procedure as in 
step 1, but at a place in the opposite 
direction along the trolley wire-rail; 
that is, 80 feet or so on the other side 
of the feed point. Keep records as 
before. 

3. Return to the first point and 
measure and record the signal level for a 
dispatcher's transmission. 

5.3.1a.ii Heaters 



WARNING 

Some oflliesL' procedures are undertaken with ihe trolley 
wire energized; therefore, they are extremely hazardous, 
txtreme caution must l)e exercised to avoid potentially 
lethal shock. The fuses used in the test leads serve only 
to ])rotect equipment and do not in any way reduce the 
shock hazard lo personnel. Only personnel thorouslily 
familiar with electrical work on troUev wires should con- 
duct these jMocedures. The permanent coinieclion of 
components should be done with power removed. Care 
should also be taken to insure that components and equip- 
ment are suitable for use in the desired application. 



FIGURE 5=13o - Permanent attachment of tun= 
ing capacitor to fixed inductor. 



as shown in figure 5-8C 
described below: 



The steps are 



1. Locate a position about 80 feet 
along the trolley wire-rail from the 
place where the rectifier feed wires 
attach to the trolley wire-rail. Install 
the test set as illustrated in figure 5-9 
between the trolley wire and rail. 
Request the dispatcher to transmit for 
about 20 seconds. Switch the capacitor 
box through its values to find a maximum 
signal level, as indicated on the tuned 
voltmeter. (The decade box should have 
enough range to peak, the voltage level. ) 
Note the value with the decade capacitor 
in the "off" position and the value to 
maximum signal, and list these values on 



Personnel heaters with a wide range 
of wattage ratings are used; however, 
1,000 watts is likely to be the lowest, 
and each heater of this rating or higher 
poses a significant signal loss to the 
carrier system. Such heaters will range 
in resistance from 360 ohms for a 1-kW 
unit on a 600-volt line to 18 ohms for a 
5-kW unit on a 300-volt line. Current 
will range from 1.5 to 17 amperes for 
corresponding conditions. Unlike recti- 
fier currents, these heater currents are 
sufficiently low that commercial induc- 
tors can be used untuned to provide 
isolation of heaters, thereby avoiding 
the step of individually tuning each 
isolator. 

To raise the impedance level to 
300 ohms at 100 kHz using an untuned 
inductance requires an inductor of 
500 pH. While it would be convenient to 
find a single inductor usable for all 
such loads, the wide range of direct 
currents that must be handled (1.5 to 
17 amperes) makes it necessary to select 
each inductor on an individual basis. 

The procedure for treating personnel 
heaters follows: Locate the heater ele- 
ment. Measure the carrier frequency 
voltage at this load, using the tuned 
voltmeter and a dispatcher's transmis- 
sion. Note this value on a mine map. 
Disconnect the heater and permanently 



137 



attach a 500-yH inductor in series with 
the element. Reconnect the heater and 
measure the voltage produced across the 
heater and inductor in series, using the 
tuned voltmeter and a dispatcher's trans- 
mission. An improvement in voltage of up 
to 10 to 1 can be expected. Note the new 
received voltage on the mine map. Repeat 
this procedure for each personnel 
heater. 

5.3.1a.iii Vehicle Lights 

Mine vehicles, including locomo- 
tives, jeeps, and portal buses, all draw 
substantial power from the trolley wire. 
Much of this power is used for motive 
purposes. Motors represent a relatively 
high impedance at trolley carrier fre- 
quencies, particularly for jeeps and por- 
tal buses and to a lesser extent for 
locomotives. However, a part of the 
power is used for headlights on the 
vehicles. Most conventional vehicles use 
150-watt, 32-volt, PAR-type lights for 
this purpose. The difference between 
32 volts and the trolley voltage is taken 
up with a ballast resistor. A single 
light circuit of this type presents a 
resistance of about 50 ohms on a 300-volt 
circuit and about 110 ohms on a 600-volt 
circuit. Because some vehicles use two 
lights at a time, and some only one, the 
bridging loads represented by the vehicle 
lights range from 110 to 25 ohms per 
vehicle. These values are sufficiently 
low that treatment is desirable. 

The procedure for treating vehicle lights 
follows: Insert a 10-ampere, SOO-yH 
inductor in series with the light circuit 
of each vehicle. Make sure that the 
inductor is only in series with the light 
circuit and is not in series with the 
motor or trolley phone circuits. Because 
of the variable conditions faced by the 
vehicles, it is not of much utility to 
check the before and after carrier fre- 
quency voltages found on vehicles, but 
the tuned voltmeter could be used for 
this purpose if so desired. 



5.3.1a.iv Other Loads 

Other loads can also adversely 
affect propagation on the trolley wire- 
rail; for example, signal and illumina- 
tion lights. As noted earlier, an indi- 
vidual light bulb, or a string of such 
lights, does not impose much insertion 
loss. However, if there are many lights, 
the total effect could be substantial. A 
way to estimate whether such lights 
affect propagation significantly is to 
count the number of lights on the trolley 
wire-rail between the dispatcher and the 
farthest place in the mine, and calculate 
the total bridging resistance. Approxi- 
mate value of loss versus bridging load 
can be estimated from figure 5-7. If the 
toal loss is less than 6 dB, only margin- 
al improvements will result from treating 
these lights. If the loss is more than 
6 dB, consideration should be given to 
treating the lights. It would be a rath- 
er unusual situation to find lights that 
really represented a significant impedi- 
ment to propagation of a trolley wire- 
rail. However, when marginal signal lev- 
els exist, the lights could well make the 
difference between marginal and fully 
usable signal levels. 

The most effective way to treat such 
lights would be to take them off the line 
and operate them from the ac system. 
This practice is being used in some of 
the newer mines. In old mines, where ac 
power is not available, little can be 
done. In some instances, the power rat- 
ing of the lights could be reduced, 
thereby raising the value of the bridging 
impedance. Fixed inductors could also be 
used but would only have small effects 
because the light strings (typically 
three 100-watt, 115-volt lights in series 
in a 300-volt system) already have a 
fairly high resistance (approximately 300 
ohms for the example above). 

Other loads are comprised of such 
equipment as pumps and other motor- 
driven devices. However, these devices 



138 



generally have high enough Impedances and 
are placed so infrequently that they re- 
sult in minimal loading effects. 

5.3.1b Using a Dedicated Wire 

As previously mentioned, the trolley 
wire-rail is an inefficient transmission 
path because of the many loads that exist 
on the line. In the dedicated-wire tech- 
nique, an independent wire (called the 
"dedicated wire") is run down the entry- 
way with the trolley line on the wide 
side, but not connected to the trolley 
line in any manner. 

Such a wire, since it is unloaded, 
has a very low attenuation rate. There- 
fore, if a signal is transmitted on the 
dedicated wire, the signal strength re- 
mains high. Since the trolley line and 
dedicated wire are located in the same 
entry, there is a mutual electromagnetic 
coupling between them. (The effects of 
loads on the trolley line are transferred 
to the dedicated wire, and the high sig- 
nal on the dedicated wire is transferred 
to the trolley line.) Fortunately, if 
the separation between the two is large 
enough (9 feet or more), the loading ef- 
fects of the trolley line are only weakly 
transferred to the dedicated wire, so 
that the attenuation rate stays low. But 
at the same time, the high signal levels 
on the dedicated wire are strongly cou- 
pled to the trolley line. The net result 
is that communication is now possible in 
areas where it was not possible before. 
The procedure for developing a system 
based on a dedicated wire is divided into 
the following three steps: 

1. Routing . — Ascertain from a mine 
map the area of coverage desired, con- 
sidering that the dispatcher position is 
the key position. Mark out on this map a 
route for a single line that runs in the 
same entryway as the trolley wire-rail to 
which communication is desired. Avoid 
branches on this route. If necessary, 
use a second or third such route to cover 
all regions of the mine. Short side- 
tracks need not be covered initially. If 
a branch on the route will cover the 
desired region with less length of wire. 



use a branch, but minimize the number of 
branches. 

2. Installation . — Install the wire; 
No. 10 or No. 12 wire is well suited to 
the task. Copper-weld construction is 
recommended for strength and integrity. 
This wire must be insulated and also held 
away from the rib or roof for at least 
3 inches. Installation must be on the 
wide side of the entry, and the wire 
should be located for least exposure to 
damage. At the far ends of each line, 
the wire is terminated by a 200-ohm, 
10-watt resistor to the rail, as illus- 
trated in figure 5-14. If branches are 
used, signal-splitting resistors must be 
included (fig. 5-15) to reduce signal 
attenuation. 

3. Connection of Transmitter. — Upon 
completion of the installation of the 







INSULATORS 




FIGURE 5-14, - Termination of the dedicated wire. 



139 



-UTILITV BOX 



FUSE HOLDERS (31 



DEDICATED WIRE 



GATED WIRE 




FIGURE 5-15. - Signal splitter. 

special-purpose wire, the dispatcher's 
transmitter should be directly connected 
to the end or ends of the wire that con- 
verge on the dispatcher's station. The 
return wire of the transmitter should go 
to earth or to the rail. 

As noted before, the use of branches 
should be minimized. When the routes are 
short, (considerably less than 10 miles), 
resort can be made to branches on a dedi- 
cated wire. When more than one wire is 
used, they should be run in separate en- 
tryways. The reason that branches are 
undesirable is that a branch reduces the 
signal level by 2 to 1 (6 dB). On short 
runs, such a loss can be tolerated, but 
on runs approaching 10 miles, such a loss 
may be too high. 

5.3.1c Using a Remote Transceiver 

Frequently the dispatcher is located 
at one edge of the mine complex. If this 
is the case, the communication range 
required must be extensive in order that 
the dispatcher be able to reach motormen 
on the opposite side of the mine. In 
some Instances, a convenient way of solv- 
ing the dispatcher's problem is to use a 
remote transceiver located at the most 
favorable place for reaching all parts of 
a mine complex. The location of such a 
remote transceiver is likely to be near 
the center of the rail haulage system of 



the mine, although in certain circum- 
stances moving it somewhat away from 
such a center might produce more favor- 
able results. 

As an example of what might be 
achieved by this means, consider a dis- 
patcher's position for which the signal 
attenuation is 80 dB from his position to 
the farthest reach of the mine. This at- 
tenuation means that an initial 25-volt 
rms signal provided by the dispatcher's 
transmitter would be reduced to 2.5 mV at 
the farthest reach of the mine. This 
level of signal is marginal, and thus the 
dispatcher would have poor communication 
to those motors on the far side of the 
mine. If the dispatcher's transceiver 
were moved to the center of such a mine, 
the signal attenuation should drop to 
one-half, or 40 dB, from this central po- 
sition to the extremities of the mine. 
The 40 dB of attenuation would provide 
signal levels of 250 mV at the extremes 
of the mine, 100 times bigger than would 
result if the dispatcher's transmitter 
were located at one edge of the mine. 

Such a substantial improvement in 
signal levels throughout the mine would 
change an otherwise marginal operation 
into a completely adequate communication 
system. Insofar as the ispatcher is 
concerned, his operation would remain the 
same. He would still have the carrier 
phone speaker and microphone located at 
his dispatching position; however, the 
control and audio signals would be trans- 
mitted from his position through a 
twisted shielded pair to the remote 
transceiver (fig. 5-16). Thus, it would 
be necessary to run an audio cable for 
whatever distance was necessary to reach 
the center of the mine. In mines where 
multipair telephone cable is used, a pair 
may be available for this purpose. If 
not, the expense and inconvenience of in- 
stalling such a cable would be justified 
to assure adequate coverage for the dis- 
patcher's communication system. 



140 



IF THE TROLLEY IS SECTI0NALI2ED, COUPLING 
CAPACITORS (5 uf. 1000 VOLTS) MUST BE USED 
TO PROVIDE A PATH FOR THE CARRIER 
COMMUNICATIONS. 



di 



DISPATCHER ' 



TRANSCEIVER 



WARNING 

Some ofLliese procedures arc underiakeii with llie trolley 
wire crieri;izcd; therefore, they are extremely hazardous, 
t'.xtreme caution must be exercised to avoid ]>olentially 
lethal shock. ITie fuses used in the lest leads serve only 
to |>rotect equipment and do not in any wav reduce ihc 
shock hazard lo personnel. Only fievsonnel thovouj;lily 
familiar widi elccirical work on irollev wires sliould con- 
duct these inoccdures. The permaneni coinieclion ol 
components shoidd be done with power renu)ved. Care 
should also be laken to instuc thai com] ion en Is and equip- 
ment aie suiiable for use in ihe desired application. 



FIGURE 5-16. - Dispatcher's remote transceivero 



5. 3. Id Summary 

A substantial number of the problems 
associated with maintaining good trolley 
communication systems can be avoided by 
advanced planning. For those planning a 
communication system for a new mine, the 
following suggestions are offered to as- 
sure optimum operation of the trolley 
carrier phone system when installed: 

1. If the trolley wire is section- 
alized, make sure capacitors (5 \i¥ , 
1,000 volts) (some systems may require 
even higher voltage components) tie the 
sections together. 

2. Plan to operate as many auxil- 
iary loads as is practicable on mine ac 
power rather than from the trolley wire 
power. 



5. Insist that vehicle manufactur- 
ers indicate the 88- to 100-k.Hz operating 
impedance of their vehicles, and select 
vehicles that show high operating imped- 
ance at the carrier frequency. 

6. If possible, use at least a 50- 
foot setback for rectifiers that are to 
be installed in the mine; this setback 
will permit tuning of the rectifier leads 
to raise the impedance of the rectifier. 

7. Ask the rectifier manufacturers 
to supply internal filters in series with 
the voltage to raise the carrier fre- 
quency impedance to a high level. 

8. Plan and design isolators for 
all other appreciable bridging loads 
across the trolley wire rail. 

5.3.2 Improving Telephone Systems 

As mentioned earlier, hardwired 
phone systems fall into three major cate- 
gories: single pair (party-line), multi- 
pair, and multiplex phone systems. A ma- 
jor disadvantage of single-pair systems 
is that each telephone must be used in a 
party-line arrangement. This prevents 
simultaneous conversations in the system 
and reduces its usefulness for discussing 
maintenance problems or other uses that 
can tie up the system for long periods of 
time. Multipair and multiplex systems 
provide for many simultaneous conversa- 
tions but until recently did not possess 
the paging ability. 

All three of these systems can usu- 
ally be improved if the basic reasons for 
poor performance or high noise levels are 
understood. For instance: 

Heavier gage wire presents less at- 
tenuation to the signal and results in 
better coverage over greater distance. 



3. Consider the use of a dedicated 
wire to aid signal propagation. 



Splicing technique has a large ef- 
fect on signal strength. 



4. Select carrier 
ers that show a high 
impedance. 



phone transceiv- 
value of standby 



Twisted pair cable can reduce noise 
pickup. 



141 



Even when proper precautions have 
been taken, all hardwired systems are in- 
herently unreliable. For example, if a 
telephone line is broken or shorted by a 
roof fall, all telephones beyond that 
point are severed from communication to 
the outside. If the line is shorted, 
communications in the entire system may 
be severely affected or lost completely. 
These deficiencies can be corrected by 
the following methods: 

Adding loopback to the phone line. 

Sectionalizing the phone system. 

5.3.2a Loopback Methods 

A major disadvantage of any wired 
phone system is its dependence upon a 
continuous phone line running throughout 
the mine. If this phone line is broken, 
communication with all phones inby the 
break is lost. Alternate communication 
paths, or loopbacks, can be established 
as shown in figure 5-17 to overcome this 
deficiency. If a line break should oc- 
cur, the loopback switch can be closed, 
allowing each and every phone to still 
communicate with all other phones in the 
system. 

Another way to implement loopback is 
to return the phone line to the main 
shaft using a different underground path. 
No matter which method of loopback is 
used, the operation of the systems is 
similar. During normal operation the 



DISPATCHERS LOCATION 



77777^ 




FIGURE 5.17. - Phoneline loopback. 



loopback switch is left in the open posi- 
tion. If a line break should occur any- 
where in the underground phone line, the 
loopback switch can be closed and each 
phone will still be able to communicate 
with other phones. Depending upon the 
physical layout of the mine, forming an 
underground loop may actually require 
less wire than if a single line is strung 
with many branches running to the indi- 
vidual phones. It is imperative that the 
loopback switch be always left open under 
normal conditions to avoid "masking" line 
breaks. 

Another method of establishing loop- 
back is by using an overland radio link. 
In this type of system the mine telephone 
signals are returned from the end of the 
line to the surface through a ventilation 
shaft or borehole. At the surface a two- 
way radio base station establishes an 
overland radio link to a second station 
near the dispatcher or general mine fore- 
man's office. Note that provisions must 
be made for dc paging. 

Each of the loopback systems de- 
scribed above utilized a loopback switch 
that during normal operation (no line 
breaks) is left in the "open" position. 
This loopback switch serves an important 
function in any loopback system. For in- 
stance, consider what would happen in 
a loopbacked system with no loopback 
switch, or if the switch is normally left 
closed. No communication outages would 
be experienced when the first line break 
occurred because each phone would still 
be connected, through one or the other 
legs of the loop, to the system. The 
problem is that unless someone under- 
ground noticed the broken phone line, 
everyone would assume that the system 
was completely intact because no com- 
munication difficulties were being exper- 
ienced. The system could operate in this 
mode for a long period of time. How- 
ever, when a second line break occurred 
communications to and from all phones be- 
tween the two breaks would be lost. Note 
also that each time the dispatcher talks 
he hears himself on the loopback phone. 
This feature alone assures that the phone 
line is intact. 



142 



5.3.2b Sectionalizing the Underground 
Network 

The desirability of selective area 
paging and simultaneous conversation 
capability along with the maximum possi- 
ble use of two-wire transmission line 
makes the use of a zoning or sectional- 
ization of the mine telephone system at- 
tractive. In this method, each zone or 
section in the underground complex is 
served by its own cable pair. 

To see how the telephone sections 
would be interconnected, consider the 
simplified four-section system shown in 
figure 5-18. Within each area, the pag- 
ing telephones would operate normally. 
That is , all phones in each 
operate on a party-line basis, 
tact with a phone outside the local area 
is desired, connection to the area being 
called would be made at an outside cen- 
tral exchange. This type of system could 
also be made more reliable by having two 
different signal paths (loopbacks) avail- 
able between each area and the central 
exchange. 



area would 
When con- 



yyr^T^ 




FIGURE 5-18. - Sectional ization of a phone system. 



5.3.3 Summary 

Advanced planning is essential to 
the successful design and installation 
of any communication system. The design 
plan should take into consideration 
changes in system requirements to meet 
communication demands throughout the en- 
tire life of the mine. 

Single-pair, multipair, and multi- 
plex systems are the basic choices avail- 
able once it has been determined that 
a hardwired system will best meet the 
communication requirements. A consider- 
able percentage of the expense involved 
in each of these systems is due to the 
distribution (cable) network, and advance 
planning is especially critical in this 
area. Wire lines to meet telemetry re- 
quirements for remote control and mon- 
itoring of equipment and atmospheric 
conditions should also be recognized. 
Note that MSHA regulations may prohib- 
it running two systems in a single 
cable. 

Methods also exist that allow im- 
provement of systems already installed. 
The performance of trolley carrier sys- 
tems can be improved by removing or iso- 
lating bridging loads on the trolley wire 
that cause signal attenuation. Dedicated 
lines or remote transceivers can also be 
used to improve the quality of these 
systems . 

General maintenance and splicing 
technique can have a large effect on the 
quality of voice service over wire phone 
systems. These systems can also be made 
more reliable by providing loopback 
paths so that each phone will remain con- 
nected to the system in case of a line 
break. 



BIBLIOGRAPHY 



143 



1. Aldridge, M. D. Analysis of Com- 
munication Systems in Coal Mines. Bu- 
Mines OFR 72-73, June 1973, 211 pp.; 
available from NTIS PB 225 862. 

2. Lagace, R. L. , W. G. Bender, J. D. 
Foulk.es, and P. F. O'Brien. Technical 
Services for Mine Communications Re- 
search. Applicability of Available Mul- 
tiplex Carrier Equipment for Mine Tele- 
phone Systems. BuMines OFR 20(1 )-76, 
July 1975, 95 pp.; available from NTIS PB 
249 829. 



3. Parkinson, H. E. Mine Pager to 
Public Telephone Interconnect System. 
BuMines RI 7976, 1974, 14 pp. 



D. 



4. Spencer, R. H. , P. O'Brien, and 
Jeffreys. Guidelines for Trolley Car- 



rier Phone Systems. 
March 1977, 170 pp. 
NTIS PB 273 479. 



BuMines OFR 150-77, 
available from 



144 



CHAPTER 6. —INSTALLATION TECHNIQUES 



6.1 The Basic Philosophy 

The investment involved in any com- 
munication system represents a consider- 
able sum. Even though it is desirable 
that the system work properly each and 
every time it is called into use, some 
failures are bound to occur. Most fail- 
ures, however, and especially those that 
occur most frequently, are due to poor 
installation techniques. An extra hour 
spent at an installation site can save 
many maintenance trips and many frustrat- 
ing hours of system troubleshooting. 

Typical faults likely to cause com- 
munication outrage are 

Pager phone systems 

Poor splices aggravated by corrosion. 

Strain relief not provided. 

Drip loop not provided. 

Incorrect branch connections. 

Overloading the circuit. 

Poor battery connections. 

Improper wire size or type. 

Lightning strikes. 

Improper placement of wire runs. 

Carrier phones 

Mounting transceiver near load resis- 
tors or other sources of heat. 

Tracks not electrically bonded. 

Cable abrasion due to poor mounting 
location. 

Disconnected battery. 

Poor mechanical installation. 



Each installation should be well 
planned. After an installation is 
completed, the technician should ask 
the question, "What can go wrong with 
this unit or line?" Remember the adage, 
"Whatever can go wrong, will." Preven- 
tive measures taken during installation 
will pay off in the long run. 

6.2 Pager Phone Installation 

The pager phones used in many under- 
ground coal mines are battery-operated, 
party-line telephones with provisions for 
loudspeaker paging. The system is usual- 
ly two-wire, nonpolarized, and operated 
by self-contained batteries. Many of the 
individual units are certified as 
permissible. 

6.2.1 Mounting 

Pager phones are designed to be 
mounted on an upright support at the 
desired location. For convenience, the 
phone should be mounted 5 feet above the 
floor where there is no obstruction to 
using the handset or removing the cabinet 
front cover for servicing or battery re- 
placement. In low-coal situations, a 
suitable height for installation should 
be selected convenient to the normal 
operator's position at the site selected. 
About 12 Inches of free space on each 
side of the phone should be provided for 
cabinet access. The phone should be pro- 
tected against direct exposure to drip- 
ping water and should not be allowed 
to rest in a puddle of water. The mount- 
ing location should be convenient to 
a work location and have a safe, unob- 
structed area for a worker to stand and 
use the phone. The phone must be in a 
location where the worker will not be in 
the path of moving vehicles or falling 
debris. Each telephone is normally well 
insulated, but it is still good practice 
to provide an insulating mat or dry 
planking for the user to stand on. 



145 



6.2.2 Connections 

For handling convenience, the branch 
line or connecting cable to each individ- 
ual telephone can be a lighter wire gage 
than the main cable. Each connection to 
the main line should be a good electrical 
and mecTianical joint, protected by a 
careful double wrap of plastic electrical 
tape. 

Special care should be taken to in- 
sure that each splice is a good electri- 
cal and mechanical connection. Connec- 
tions that are of poor or marginal 
quality, or that are not adequately pro- 
tected from moisture, will contribute to 
poor performance. During periods when 
humidity levels are high, especially dur- 
ing the summer months, corrosion will 
form on all exposed splices. As this 
corrosion builds, audio levels decrease 
and line noise increases until eventually 
the entire system becomes useless. 

Connections at the phone depend on 
each manufacturer's design and on indi- 
vidual state or local requirements. A 
majority of the phones provide two ex- 
posed spring-loaded terminals for attach- 
ing the wires. For proper connection, it 
is necessary to strip the installation 
away from each conductor in the pair, 
seal off the exposed area of the cable 
with plastic electrical tape to keep out 
moisture, and then insert one of the ex- 
posed conductors into each of the cabinet 
terminals. Some states, such as Pennsyl- 
vania, do not allow the use of exposed 
terminals at the face area of gassy 
mines. For these applications, some 
phones are equipped with twist-lock con- 
nectors at the end of a short cable. 
Each connector is mated with a similar 
connector on the drop or branch line to 
complete the installation. In either 
type of installation, there should be a 
drip loop below the cabinet to prevent 
condensate from running down the cable 
into the cabinet. 

6.2.3 Batteries 

Pager phones are usually operated by 
one (or two) 12-volt, dry-cell batteries. 



NEDA No. 923 or No. 926 (National Elec- 
tronic Distributors Association) . To in- 
stall batteries, it is necessary to open 
the pager phone cabinet and inspect the 
battery compartment. Remove the old bat- 
tery by loosening the retaining clamp, 
and either unscrew the battery terminals 
to release the battery wires or remove 
the battery plug, depending on the bat- 
tery type. Remove the battery, and care- 
fully wipe out the battery compartment to 
remove dirt and moisture. Place a fresh 
battery in the compartment, and secure 
the retaining clamps tight enough to re- 
strain the battery without crushing or 
bending the battery case. Reconnect the 
battery wires , being careful to observe 
the polarity markings noted on the case. 
If the plug-connector type is used, do 
not force the connector. Correct polar- 
ity is maintained when the larger connec- 
tor pin fits in the larger hole. The 
difference in pin sizes is not great, so 
a mismatch can be forced. If the connec- 
tor does not mate easily, reverse it and 
try again without forcing. After replac- 
ing the battery, close the cabinet and 
mark the date of battery replacement 
either on the outside of the cabinet or 
in a log book. 



CAUTION 

Pager phone circuits are normal- 
ly designed to provide sufficient 
current limiting with the specified 
battery. If other battery types are 
used, such as the nickel-cadmium re- 
chargeable type or one of the alka- 
line, long-life, high-current vari- 
eties, the circuit may not be able to 
limit the available current to a safe 
value. REPLACE WITH RECOMMENDED 
BATTERY ONLY. 



Battery life is not easy to predict, 
because of the many operating variables 
that affect the average current drain. 
In general, the batteries in a telephone 
system that is used many times a day may 
have to be replaced every 4 to 6 weeks , 
while a telephone system that is seldom 
used may keep its batteries at usable 
strength for 4 to 6 months. 



1A6 



Each battery change should be re- 
corded, either on the telephone cabinet 
or in a central log. Experience gained 
over a period of time will help predict 
when a battery in a particular phone is 
reaching the end of its useful life. 
Periodic verification of battery status 
at each phone should be made with a volt- 
meter and recorded in the log. (Measure 
battery voltage while under load; that 
is, during paging.) When the battery 
voltage drops to a value that is 75% to 
80% of the installed level, it should be 
replaced. For example, for a 12-volt 
battery, the replacement level is about 
8 to 9 volts. 

6.2.4 Fuses 



6.3 Phone Lines and Transmission Cables 

6.3.1 Phone Lines 

The cable used to interconnect un- 
derground pager phones must be rugged 
enough to withstand the underground envi- 
ronment and also have the proper elec- 
trical characteristics for requirements 
of the pager system. Generally, the 
cable used for this purpose is a twisted 
pair of solid-conductor wires that has a 
nonwater-absorbing, flame-retardant insu- 
lation with a rating of 600 volts dc and 
an outer abrasion-resistant covering. 
The conductor used depends on the instal- 
lation; recommended sizes are 19 AWG to 
14 AWG. 



Fuses are provided in pager tele- 
phones as an added precaution against ex- 
cessive current in the external circuit. 
Current-limiting circuitry is normally 
provided in the telephone, but the fuse 
is an additional safeguard. No provision 
is made in most phones to store a spare 
fuse. It is good practice to tape two 
additional spare fuses to the inside of 
the cabinet when the phone is first in- 
stalled. Then, the correct fuse will be 
available at the phone if it is needed. 
Make sure the fuses do not and cannot 
short circuit any circuitry. 

6.2.5 Amplifier Loudness 

Each pager phone has a loudspeaker, 
powered by its own internal amplifier, 
that is switched on by the dc paging sig- 
nal. The available audio power is about 
5 watts, which is adequate to be heard 
above most mine noises. Many telephones 
have a volume control for the speaker. 
During installation, the speaker should 
be oriented, and the volume set, to in- 
sure adequate coverage in the area. 

During setup, someone should page 
from another location to the phone being 
installed. The volume control should be 
set to the desired level during the pag- 
ing. The telephone cabinet should be 
positioned to direct maximum sound to 
the work area. 



Many telephones used underground, 
particularly those used at the working 
face, are subject to periodic relocation. 
To allow for this , and to reduce the 
problems associated with repeated cable 
splicing, some convenient length of wire 
(say, 500 feet) can be included as part 
of the branch line. This extra wire can 
be kept reeled, or neatly coiled in a 
bundle and secured with a few wraps of 
plastic electrical tape. The extra wire 
should be hung near the telephone in a 
place free of dripping water or water ac- 
cumulation, and should be supported by an 
insulated hanger that is isolated from 
power or trolley wires. The practice of 
coiling the cable is recommended, but 
with certain restrictions. If the cable 
is used to transmit monitor signals via 
an RF (radio frequency) carrier imposed 
on the two-wire pager phone line, the 
coiled cable becomes an Inductor that 
will impede the proper transmission of 
the RF signal.'' 

Methods and recommended techniques 
for the permanent installation of phone 
lines are presented in a Bureau of Mines 

^ The addition of equipment to a phone 
system could violate intrinsic safety 
standards; check with MS HA for detailed 
application information. 



147 



handbook (2^) ,2 including installation, 
lightning protection, cable selection, 
and splicing methods. Note that 30 CFR 
specifies certain requirements for cable 
installation. 

6.3.2 Leaky Feeder Cable 

Installation of leaky feeder cable 
requires some special techniques. A typ- 
ical installation of leaky feeder cable 
is shown in figure 6-1. For installation 
of the repeaters, refer to the manufac- 
turer's installation guide. Hanging the 
leaky feeder cable requires clamps such 
as those used for conduits or other power 
cables. In areas where corrosion may be 
a problem, stainless steel or plastic 
clamps should be used. Typical hangers 
are shown in figure 6-2. The type of in- 
sulated hanger shown supports the leaky 
feeder cable from the messenger cable. 
Leaky feeder cable should be supported at 
intervals of 5 feet and is usually termi- 
nated with an antenna. 

6.4 Carrier Phone Installation 

The primary function of the carrier 
phone system is to provide a reliable 
communication network over which the dis- 
patcher can direct all tracked vehicle 

^Underlined numbers in parentheses re- 
fer to items in the bibliography at the 
end of this chapter. 





REPEATEH 


^SPUTTER 




flEPEATEH 




,...., 






^ 


X 




_^J^ 








REPEATER 


SPACING 










AfiOOF 


EET 







traffic in the mine. The safety and pro- 
ductivity of the mine depend, to a large 
measure, on the ability of the dispatcher 
to maintain direct contact with all mo- 
tormen via the carrier phone system. For 
this reason, the carrier phone installa- 
tion should be carefully thought out, and 
the workmanship should be of the highest 
caliber. 



CAUTION 

Installation procedures in this 
section are guidelines and not com- 
prehensive technical instructions. 
Procedures described in this section 
must be performed by people thorough- 
ly qualified to do such work. In- 
stallations should comply with manu- 
facturer's recommendations, good 
safety procedures, and all applicable 
codes and regulations. 



FIGURE 6-1. - Typical installation. 



6.4.1 The Dispatcher Location 

The trend in modern coal mining is 
to locate the dispatcher aboveground in a 
separate building or a separate room in 
the mine office complex. This location 
provides a continuously manned communica- 
tions center even if the mine must be 
evacuated owing to emergencies or venti- 
lation failures. Since 1974 the mining 
laws of West Virginia have required that 
the dispatcher be located on the surface 
in all new mines and for existing mines 
if the dispatcher is relocated (Article 
22-2-37, Part T5). 

Underground dispatchers' locations 
vary greatly , depending on the mine lay- 
out and growth. The two most common lo- 
cations chosen are at the bottom of the 
main shaft or near the physical center of 
the mine. 





INSULATED MESSENGER 
CABLE HANGER 



METAL HANGER 

FIGURE 6=2. = Hanger hardware 



The carrier phone equipment is usu- 
ally installed on a panel which is 
mounted on a wall adjacent to the dis- 
patchers' desk. This panel provides one 
convenient location for all the subassem- 
blies that make up a carrier phone and 
protects the interconnecting cables 
from unnecessary flexing and stretching. 
The panel should be made from at least 



148 



1/8-inch-thick steel plate if the carrier 
phone uses a power-conditioning unit or 
resistor box that contains series- 
dropping resistors. 



CAUTION 

Remove the electronic subassem- 
blies from the mounting plate during 
welding operations. Keep all elec- 
trical cables and other nonmetallic 
materials away from the welding area. 
This will prevent the carrier phone 
components from being damaged by heat 
during welding operations. 



The microphone-speaker assembly is 
the only part of the carrier phone that 
interfaces directly with the dispatcher; 
therefore, it must be located within easy 
reach. The speaker volume control should 
also be within easy reach. 

Locate the transceiver on the panel 
at either side of the speaker assembly, 
taking into consideration the location of 
the interconnecting cables. Leave room 
for the excess cable to be coiled up and 
secured to the panel. 

Temperature-sensitive electronic 
circuits are located inside the trans- 
ceiver assembly. Therefore, it should be 
protected from the temperature extremes 
produced by load resistor banks and room 
heaters. For reliable operation, the am- 
bient operating temperature range that 
the transceiver is exposed to should be 
restricted to -40° to +140° F. 



transceiver electronics and to recharge 
the battery. The circuit generally used 
in this unit contains a large series- 
dropping resistor that under normal oper- 
ating conditions dissipates several 
hundred watts. The high temperature as- 
sociated with this power dissipation 
would be harmful to the sensitive trans- 
ceiver circuitry; therefore, it is a 
separate unit that can be located where 
it will not heat up the transceiver. 
When only the series-dropping resistor is 
contained in this unit, it is called a 
resistor box. It is also referred to as 
the battery charger by some manufactur- 
ers; in this case, it would contain the 
dropping resistors and the charging 
circuits. 

The main consideration when locating 
this unit is its heat dissipation and its 
relationship to the heat-sensitive trans- 
ceiver. The heat is dissipated into the 
ambient air and into the structure on 
which it is fastened; therefore, it is 
important to follow the manufacturer's 
mounting instructions carefully. 

The power unit should never be 
mounted below the transceiver (heat 
rises) or the speaker enclosure. Keep 
the power unit a minimum of 6 inches away 
from either side or the top of the trans- 
ceiver. If mine personnel can come in 
contact with the hot surfaces of the 
power-conditioning unit, a protective 
grille should be added. This grille 
should be open at the top and bottom to 
allow for proper air circulation. 



Approximately 6 inches of clearance 
should be left around all surfaces on 
which the connectors and/or fuses are 
mounted. If possible, the connector- 
mounting surfaces should be protected 
from dirt and moisture. Sufficient 
clearance should be allowed to remove ac- 
cess covers and open-hinged panels so 
that adjustments can be reached and plug- 
in modules can be changed. 

The power-conditioning unit is used 
to convert the trolley voltage (typical- 
ly 300 or 600 volts dc) or the local 
ac power to 12-volt dc power for the 



A 12-volt lead-acid automotive-type 
storage battery is most often used as an 
external emergency power source with car- 
rier phones. When locating this type of 
battery, the prime considerations should 
be the accessibility of the fill caps for 
servicing and proper room ventilation to 
handle the outgassing of hydrogen. The 
battery should also be kept away from ma- 
terials that are susceptible to corrosion 
by sulfuric acid. 

The ideal temperature range for the 
battery is 60° to 80° F. Low tempera- 
tures reduce capacity but prolong battery 



149 



life; high temperatures give some addi- 
tional capacity but reduce total battery 
life. Temperatures above 125° F can ac- 
tually damage some of the battery compo- 
nents and cause early failure. 

Once the various subassemblies have 
been physically mounted to the panel, the 
final installation task is to make the 
electrical interconnections. This pro- 
cedure consists primarily of inserting 
cable-mounted connectors into the proper 
receptacles on the subassemblies and con- 
necting the signal and power cords into 
the proper mine ectrical systems. 

A block diagram of a typical carrier 
phone interconnecting cable system is 
shown in figure 6-3. The cable connected 
to the trolley power and/or building pow- 
er should be installed last. The other 
cables may be installed in any order that 
is convenient. 



CAUTION 

Clean and inspect all connectors 
before mating. Study the keying ar- 
rangement or polarization to prevent 
jamming and misalinement. 



Before connecting power, verify that 
the RF signal common and the case and 
chassis grounds are all connected. The 
RF signal common should be an all- 
metallic connection to the rail system, 
even if the dispatcher is located above- 
ground. Often the rails are bonded to 
the steel structural members of the main 
shaft to help establish a good earth 



POWER 
AND 
SIGNAL 
CABLE 



BATTERY 

CHARGER 

AND 

RESISTOR 

BOX 



» TRANSCEIVER 



<^ 



MICROPHONE 



FIGURE 6-3. - Typical carrier phone intercon- 
necting cable system. 



ground for the mine. If this is the 
case, the RF signal common can be wired 
to the shaft structure at the surface or 
the hoist house structure. A minimum 14 
AWG insulated copper wire should be used 
for this purpose. If the input power is 
supplied by the trolley wire, then the RF 
signal common and the power common should 
be jumpered together. 

Connect all chassis and case grounds 
from the lugs or studs provided by the 
manufacturer to earth ground. Do not 
rely on the mechanical mounting of the 
case for a ground connection; always run 
a separate ground wire to the earth 
ground. Refer to the Code of Federal 
Regulations, Title 30, Part 75, Subpart 
H, for explicit grounding requirements. 

The earth ground connection or 
building is generally made to a metallic 
water supply pipe or to the structural 
ironwork of the building. In either 
case, the connection should be made close 
to where the pipe or structure enters the 
earth to insure a minimum resistance be- 
tween the connection and the earth. 

The input power to the dispatcher's 
phone is supplied from either the trolley 
wire (typically 300 to 600 volts dc) or 
the local 115-volt ac power. If trolley 
wire input power is to be used with the 
dedicated line coupling method, then the 
in-line fuse holder cable is connected to 
only the hot input power terminal on the 
phone. The other end of the cable is 
connected to the trolley wire. Whenever 
trolley wire input power is used, the 
common power connection and the common 
signal connection are jumpered together 
and wired to the rails. 

If the carrier phone is not located 
adjacent to the haulageway, then a wall- 
mounted fuse box should be used instead 
of the in-line fuse. Terminate the wire 
on the line side of the fuse block. Us- 
ing the same type of wire, make a welded 
connection to the rail and run this back 
to the fuse box. Now, two-wire neoprene- 
jacketed-type portable cable may be 
used to supply power to the dispatcher's 
phone. 



150 



NOTE 

The manufacturer's detailed in- 
stallation instructions should be 
carefully followed to make certain 
the carrier phone is compatible with 
the polarity of the mine trolley 
power. 



The 12-volt power fuse and the trol- 
ley power fuse should be removed to per- 
mit making the battery connections with- 
out a load immediately being placed 
across the battery. The grounded side of 
the battery should be connected first. 
If the mine has a positive trolley sys- 
tem, then the negative side of the bat- 
tery should be grounded. The battery 
post is made of lead, as are the internal 
connections between the post and the bat- 
tery plates. If too great a torque is 
applied to the clamping bolt, the inter- 
nal connections can develop hairline 
fractures that can cause an intermittent 
connection. To avoid this condition, a 
second wrench should be used to steady 
the bolthead while tightening the nut. 

Fuses provide an intentionally weak- 
ened part of an electric circuit and 
thereby act as a safety valve in the 
event of dangerous overloads. This pro- 
tects both personnel and equipment from 
potential fire hazards due to overheating 
of the carrier phone. 



NOTE 

Fuses do not provide protection 
from dangerous high-voltage shocks. 



Fuses come in many sizes, types, and 
electrical ratings. Always use a re- 
placement fuse that has the same rating 
as specified by the carrier phone 
manufacturers . 

The last step in installation is to 
connect the power wire. Two commonly 
used installation methods are direct cou- 
pling to the trolley wire, and single 
dedicated line coupling. For the in- 
staller's safety, the input power should 
be connected last. 



Direct coupling involves wiring the 
hot RF signal connecting point directly 
to the trolley wire with the in-line 
holder cable provided with the phone. If 
the dispatcher's office is remotely 
located, then a fuse box adjacent to the 
trolley wire should be used. If the in- 
put power to the phone is to be supplied 
by the trolley wire typically (300 to 600 
volts dc) , then the hot power connection 
is jumpered to the hot RF signal connec- 
tion with a length of 14 AWG insulated 
copper wire. Do not install the 3-ampere 
in-line power fuse until all ground con- 
nections are made up. 

A second method of signal coupling 
is to connect the dispatcher's phone to a 
single conductor dedicated wire. This 
wire would originate at the hot RF signal 
connecting point. 

6.4.2 Vehicle Installations 

The carrier phone typically consists 
of a transceiver assembly, a microphone- 
speaker assembly, and power conditioning 
units; these are sometimes an integral 
part of the transceivers (fig. 6-4). 
Carrier phone equipment is installed on 
all types of tracked vehicles. Three 
commonly used vehicles found on coal 
haulage systems are locomotives, portal 
buses, and utility cars. Each of these 
vehicles has a different seating arrange- 
ment for the driver (fig. 6-5). 

The microphone-speaker assembly is 
the only part of the carrier phone that 
interfaces directly with the vehicle 
operator. Thus, it must be located so 
that it can be easily reached. If the 
microphone hanger is not conveniently lo- 
cated, it will not be used by the opera- 
tor, and the microphone and cord will 
suffer unnecessary damage from mistreat- 
ment. The speaker volume control should 
also be within easy reach, and the speak- 
er should be pointed directly at the 
operator to provide the best reception. 

Vehicles without dual controls re- 
quire the operator to assume two dif- 
ferent positions in front of the same 



151 




TRANSCEIVER 



TRANSCEIVER 



FIGURE 6-4. - Typical carrier phones. 



controls so that he can observe the track 
ahead of him. This further complicates 
the positioning of the microphone-speaker 
assembly. It is sometimes helpful to use 
two microphone hangers for this type of 
installation so that the microphone is 
convenient no matter which way the vehi- 
cle is traveling. 

Entanglement of the microphone cord 
with other vehicle controls, causing 
an unsafe operating condition, should 
also be considered when locating the 
microphone-speaker assembly. The micro- 
phone should also be mounted in an area 
that will protect it from falling debris 
and/or dripping water. 

Once a suitable place for the 
microphone-speaker assembly has been de- 
termined, the transceiver location can be 
considered. The first restriction on its 
location is the length of the cables run- 
ning between the different assemblies. 
Temperature-sensitive electronic circuits 



are located inside the transceiver assem- 
bly. Therefore, it should be protected 
from temperature extremes such as those 
produced by load resistor banks and the 
vehicle's drive motors. 

It is important that installation of 
the transceiver does not reduce the mini- 
mum roof clearance of the vehicle. Ap- 
proximately 6 inches of clearance should 
be left between the vehicle and the sur- 
faces on which the connectors and/or 
fuses are mounted. If possible, the con- 
nectors should be protected from dirt and 
moisture. Sufficient clearance should be 
allowed to permit removal of access cov- 
ers and open-hinged panels so that ad- 
justments can be reached and plug-in mod- 
ules can be changed. 

The main consideration when locating 
the power conditioning unit is its heat 
dissipation and its relationship to the 
heat-sensitive transceiver. The heat is 
dissipated into the ambient air and into 



152 




SPEAKER 
MICROPHONE 
TRANSCEIVER 

(B) UTILITY CAR 



(-SPEAKER 

I pMICROPHONE 

(ROOF REMOVED 
TO SHOW INTERIOR) 



TRANSCEIVER 

MICROPHONE 
SPEAKER 




(C) PORTAL BUS 

WITH DUAL CONTROLS 



FIGURE 6-5. - Typical mine vehicles. 



153 



the structure on which it is fastened; 
therefore, it is important to follow the 
manufacturer's mounting instructions 
carefully. The mounting surface should 
be a massive structural part of the vehi- 
cle that can absorb the heat transferred 
from the unit. 

If it is a horizontal surface, a 
minimum of 3 inches should be allowed on 
all sides; if possible, nothing should be 
mounted above the unit. If it is a ver- 
ticle surface, a minimum clearance of 3 
inches above and below the unit should be 
provided for proper air circulation. The 
power unit should never be mounted below 
the transceiver; if possible, a minimum 
separation of 1 foot in all other direc- 
tions should be provided. 

A 12-volt lead-acid automotive-type 
storage battery is most often used as an 
external emergency power source with car- 
rier phones. When locating this type of 
battery, the prime considerations should 
be the accessibility of the fill caps for 
servicing and proper room ventilation to 
handle the outgassing of hydrogen. The 
battery should also be kept away from ma- 
terials that are susceptible to corrosion 
by sulfuric acid. 

The ideal temperature range for the 
battery is 60° to 80° F. Low tempera- 
tures reduce capacity but prolong battery 
life; high temperatures give some addi- 
tional capacity but reduce total battery 
life. Temperatures above 125° F can ac- 
tually damage some of the battery compo- 
nents and cause early failure. 

Most carrier phone components are 
supplied with mounting plates that can be 
tack-welded to the vehicle. This pro- 
vides a permanent mounting surface with 
tapped holes or threaded studs onto which 
the subassemblies are fastened. This ar- 
rangement also provides an easy means of 
interchanging subassemblies for mainte- 
nance purposes. 



CAUTION 

Remove the subassembly from the 
mounting plate during the welding 
operation. Keep all electrical ca- 
bles and other nonmetallic materials 
away from the welding area. This 
will prevent the carrier phone compo- 
nents from being damaged by the heat 
generated from the welding operation. 



Procedures for making the electrical 
connections between carrier system compo- 
nents are similar to those for the dis- 
patcher's installation (paragraph 6.4.1). 
For the installer's safety, the trolley 
shoe should be removed from the trolley 
line. 

Proper cable protection will reduce 
the downtime of the communication system 
and prevent accidents , such as loose ca- 
bles tripping up mine personnel when en- 
tering or leaving the vehicle. The in- 
terconnecting cables should be located, 
if possible, away from areas occupied by 
mine personnel or supplies. This will 
prevent cutting and crushing of the ca- 
bles caused by shifting loads. 

Heavy-duty plastic ties or cable 
clamps should be used to lash the cord to 
the frame of the vehicle. If possible, 
the cable should be run under overhanging 
parts of the frame to protect it from 
falling debris and/or dripping water. 
Enough slack should be left to form a 
drip loop to prevent condensate from run- 
ning down the cord and into the rear of 
the connector. All holes in the frame 
through which the cable runs should be 
grommeted. The cable should not be run 
over sharp edges that might abrade it. 
Excess cable should be neatly coiled and 
secured with plastic electrical tape or 
cable ties and then clamped to the frame. 
The cable should never be stretched be- 
tween clamps ; this will leave it in ten- 
sion, causing an elongation of the insu- 
lation and the conductor. In addition. 



154 



the jacket will lose a considerable part 
of its resistance to mechanical damage, 
making it vulnerable to cutting, tearing, 
and abrasion. 

6.5 Carrier Current Hoist Phone 

Carrier current hoist phones utilize 
existing physical conductors (the hoist 
rope) for a transmission medium. Typical 
hoist radio hardware is shown in figure 
6-6. 



6.5.1 Cage 

The cage equipment consists of the 
transceiver, which contains a speaker, 
microphone, and push-to-talk switch, a 
battery, generally of the lead-acid type, 
a cage coupler, and the connecting ca- 
bles. The transceiver is the only unit 
that must be mounted within the cage, 
where space is usually at a premium. For 
that reason, it should be recessed in the 
cage wall. The battery must be mounted 



MICROPHONE 
\ 



PUSH-TO-TALK FOOT SWITCH 




HEADFRAME 
COUPLER 



HOIST ROPE 



POWER SUPPLY 



SPEAKER 
GRILL 



MICROPHONE 
PUSH-TO-TALK SWITCH 




COUPLER CABLE- 
FIGURE 6-6. - Hoist radio hardware. 



CAGE COUPLER 



155 



in an upright position either inside the 
cage or on top. Mounting the battery on 
the top of the cage provides early access 
for charging or replacement. The cage 
coupler is bolted to the hoist rope above 
the cage with the conical section up to 
act as a rock shield. It is suggested 
that the battery be placed within a pro- 
tective enclosure to prevent a short cir- 
cuit which could be caused by debris. Be 
sure the cable and unit connectors are 
clean before mating them. Conductive 
dust in a connector interface may cause 
the equipment to malfunction. Cables be- 
tween the battery, transceiver, and cou- 
pler should be strategically placed to 
avoid damage. Cable clamps should be 
used to take up slack; a loose cable is a 
hazard to personnel and equipment. It is 
suggested that cables be run through 
heavy-gage conduit. 

6.5.2 Hoistroom and Headframe 

The hoistroom equipment is shown in 
the upper part of figure 6-6. The hoist- 
room will contain the power supply, 
transceiver, push-to-talk foot switch, 
and microphone. The power supply and 
transceiver may be wall mounted. It is 
best to leave at least 6 inches between 
the power supply and transceiver, and the 
power supply should not be mounted below 
the transceiver. The microphone should 
be placed so that it can be within 2 
inches of the operator's mouth while the 
operator has both hands on the hoist con- 
rols. The foot switch should be in easy 
reach of the operator's foot while the 
operator is at the controls. 

The headframe coupler is located at 
the top of the shaft. It may be clamped 



to or suspended from the headframe struc- 
ture. The coupler cable from the head- 
frame coupler to the transceiver should 
be run through conduit. 

6 . 6 Summary 

Tables 6-1 through 6-4 are basic 
checklists for four types of installa- 
tion: Pager phone, carrier phone, hoist 
phone, and cable. It is evident that not 
all criteria are covered in these basic 
checklists. Additional items that are 
peculiar to a specific installation may 
be added. 

There are some procedures associated 
with any installation. They are 

READ INSTRUCTIONS BEFORE STARTING! 

DO IT SAFELY! 

DO IT CAREFULLY! 

CHECK IT THOROUGHLY! 

SEEK HELP IF NECESSARY! 

Short cuts in installation will 
probably lead to equipment malfunction or 
damage. All communications equipment 
should be tested with transmissions to 
and from another unit after installation. 

A little extra time spent in the in- 
stallation phase of a communication sys- 
tem can mean the difference between a 
reliable, well-managed system and an 
undependable system requiring frequent 
maintenance. 



156 



TABLE 6-1. - Pager phone installation basic checklist 



Item 


Yes 


No 


Comments 


1. Has the exposed area of the cable been sealed with electrical 
tape to keep out moisture? 








2. Is the drip loop positioned properly to keep condensate from 
getting into phone? 








3. Is the tension in the spring terminals sufficient for a good 
connection between the wire and phone? 








4. Is the twist-to-lock connector in the locked position? 








5. Has the battery been tested? 








6. Have spare fuses been provided? 








7. Has the phone been called from a distant phone and been found 
operable? 








8. Is the volume satisfactory or has the amplifier been adjusted 
for proper loudness? 








9. Is the phone mounted at a proper height for convenience? 








10. Is an insulating mat or dry planking provided on which the 
user may stand? 








11. Is all cabling secured and protected from passing machinery? 








12. Are all splices QUALITY splices? 








13. Are the cables heavy enough? 









TABLE 6-2. - Carrier phone installation basic checklist 



Item 


Yes 


No 


Comments 


1. Is the microphone-speaker assembly within easy reach? 








2. Is the transceiver protected from temperature extremes (away 
from power conditioning unit)? 








3. Is the power conditioning unit mounted so personnel will not 
come in contact with it? 








4. Is the power conditioning unit covered by a protective grille? 








5. Has the battery been tested under load? 








6. Have mating surfaces of connectors been inspected and 
cleaned? 








7. Have all cable connectors been firmly joined to the units? 








8. Are threaded connectors tightened? 








9. Have all chassis and case grounds been wired to earth ground? 








10. Can the microphone cord become entangled in the vehicle 
controls? 








11. Are cables protected from abrasion? 








12. Are all components mounted low enough so that minimum roof 
clearance has not increased? 








13. Has sufficient clearance been given to allow easy removal of 
access covers, hinged panels, etc.? 








14. Are the bridging capacitors in place on all sectionalized 
trolley lines? 









157 



TABLE 6-3. - Hoist phone installation basic checklist 



Item 


Yes 


No 


Comments 


1. Is the microphone of the transceiver at a proper height 
(mouth level) for average person? 








2. Is the battery accessible for charging or replacement? 








3. Is the cage coupler firmly mounted? 








4. Have mating surfaces of connectors been inspected and 
cleaned? 








5. Have cable connectors been firmly joined to the units? 








6. Are threaded connectors tightened? 








7. Are cables protected from damage? 








8. Have any slack cables been tied down (clamped)? 








9. Is headframe coupler firmly connected to or suspended from 
headf rame? 








10. Are microphone and push-to-talk switch near hoist controls? 








11. Is the transceiver frame firmly attached to the cage? 








12. Have all completed connections been sprayed with silicone or 
other moisture-inhibiting spray? 








TABLE 6-4. - Telephone cable installation basic checklist 


Item 


Yes 


No 


Comments 


1. Has proper cable been selected according to system plan (type 
and gage)? 








2. Is the cable supported at the proper interval (approximately 
10 feet for twisted pair or figure-8 cable and 5 feet for 
leaky feeder)? 








3. Do droplines have strain relief on main line and tap line? 








4. Are splices mechanically sound and protected from moisture? 








5. Is strain relief used at splices? 








6. Have lightning arrestors been used according to code? 








7. Has extra insullation been provided where the cable crosses 
the trolley or other high-voltage lines? 








8. Is the cable positioned out of the way of machinery and 
secured in place? 








9. Are all splices QUALITY splices? 









158 



BIBLIOGRAPHY 



1. Long, R. G. Guidelines for Instal- 
lation, Maintenance and Inspection of 
Mine Telephone Systems. BuMines OFR 116- 
78, June 1975, 53 pp.; NTIS PB 287 641. 

2. Long, R. G. , R. L. Chufo, and R. A. 
Watson. Technical Guidelines for In- 
stalling, Maintaining, and Inspecting 



Underground Telephone 
Handbook, 1978, 44 pp. 



Systems. BuMines 



3. Spencer, R. H. , P. O'Brien, and 
D. Jeffreys. Guidelines for Trolley Car- 
rier Phone Systems. BuMines OFR 150-77, 
March 197, 170 pp.; NTIS PB 273 479. 



CHAPTER 7.— MAINTENANCE 



159 



7.1 General 



7.2.2 Pager Phones 



Preventive maintenance practices 
are listed in the manufacturers' instruc- 
tion books. In general, equipment that 
requires frequent and extensive preven- 
tive maintenance is generally the most 
costly. The manpower spent on these fre- 
quent trips to remote areas is such that 
it is usually better to invest in a more 
costly system which requires little pre- 
ventive maintenance. The best preventive 
maintenance for the system is a good 
installation. 

7.2 Preventive Maintenance and 
Inspections 

With any system, periodic inspection 
is required because of the corrosive at- 
mosphere and adverse conditions that ex- 
ist in underground mines. These inspec- 
tions can spot potential trouble in the 
system. Repair or replacement at that 
time averts the possibility of losing ef- 
fectiveness of all or part of the system. 

7.2.1 Cables 

Approximately once per month all ca- 
bles in the communications system should 
be inspected for kinking, chafing, crack- 
ing, wear, stretching, or other signs 
of physical abuse. Particular attention 
should be paid to cable glands at the en- 
try or exit points to the various units 
in the system, where the cable goes 
around sharp corners , in the vicinity of 
holding cleats which may be clamping the 
cable too tightly causing potential dam- 
age, and across the areas where the cable 
is exposed to physical damage from out- 
side sources, such as equipment or fall- 
ing objects. If a cable is damaged, it 
should be replaced as soon as possible. 

It is mandatory that the ground 
leads and connections to carrier current 
phones be thoroughly inspected and main- 
tained in good condition, since consider- 
able hazard may exist to the operator 
or equipment if a ground connection is 
broken. 



The most readily available test set 
to determine if a pager telephone is 
operating correctly is the pager phone 
itself. The following physical check of 
the system can be performed at any phone 
station. 

7.2.2a Listen Circuit 

Remove the handset from its cradle 
and listen to determine if the circuit is 
functional, as indicated by the presence 
of noise or conversation on the line. If 
no noise or voice signals are present at 
the handset receiver, take the following 
corrective action: 

1. Operate the handset press-to- 
talk switch several times. Any corrosion 
on the contacts of this switch may cause 
a receiver to be temporarily inopera- 
tive. Repeated operation may clear the 
condition. 

2. Open the cabinet and see if the 
battery cables are properly connected and 
are making firm contact. Check the hand- 
set cable and its connections in the cab- 
inet of the pager phone, and see if there 
is any evidence of a break in the cable, 
corroded contacts, or poor connections. 

3. Remove the handset receiver ear- 
piece by unscrewing it counterclockwise 
(to the left), and remove the receiver 
from the socket. Examine the handset 
cavity; in some units, a patch of cotton 
batting or floss is used as a barrier to 
reduce acoustic feedback in the handset. 
If the cotton patch has absorbed mois- 
ture, remove it and replace with a 
crumpled ball of soft rubber, stuffed 
just far enough into the handset so it 
will not touch the receiver or switch 
terminals. 

7.2.2b Page Circuit and Talk Circuit 

Push the page switch, squeeze the 
handset press-to-talk switch, and call 
any other phone. Release the page switch 



160 



and listen for a reply. If the back- 
ground noise is too high, or if the re- 
ceived signal is either too weak or too 
garbled to be understood, then repairs 
should be initiated to improve that par- 
ticular telephone. The phone should be 
replaced by an operable unit and repaired 
by qualified personnel. Ask the answer- 
ing party if the paging signal could be 
understood, and also check the quality of 
your received signal. Have the other 
party page you to verify that your speak- 
er works. Some of the most common prob- 
lems with the pager phone system that can 
be remedied by good installation and 
maintenance practics are: very low voice 
levels and very high noise levels. 



In the environment of underground 
mines, switch contacts are particularly 
susceptible to erratic operation because 
of corrosion or oxidation of the switch- 
ing contacts. This is particularly true 
of contacts that are used infrequently. 
Repeated operation of each of the 
switches in the telephone may aid in 
clearing some of the corrosion and 
restoring the phone to more reliable 
operating condition. Cleaning individual 
contacts should not be attempted with a 
phone in service; it should only be done 
by trained or experienced repair person- 
nel, who have approved burnishing tools 
specifically designed for use on switch 
contacts. 



Often, the sources of these problems 7. 2. 2d Battery Condition 



are — 



1 . Poor placement of the phone line 
(near rectifiers, motors, etc.). 

2. Using too light a gage phone 
line. 

3. Using the wrong kind of wire for 
the phone line. The line should be of 
the twisted two-wire type. Nontwisted 
line of any gage is not acceptable. It 
is the twist that provides a great deal 
of noise immunity. 



4. Poorly made splices. 



These 



cause high resistance and leaky joints in 
the line that lower the signal and in- 
crease the noise. (Note that the phone 
systems always are worst in the summer 
months. This is because the high humid- 
ity is affecting the splices.) 

7.2.2c General Comments 

If any of the signals is erratic, 
low in signal level, has exceptionally 
high noise levels, or is unintelligible, 
check whether other phones in the system 
are having similar problems. If not, re- 
place the defective phone with a good 
one. If the other phones are not operat- 
ing properly, it is possible that the 
problem is in the line. A cable may be 
short-circuited, improperly spliced, or 
running too close to noise-producing 
power or trolley lines. Such conditions 
should be corrected. 



The battery condition of a pager 
phone can be approximately checked by 
pushing the page button and calling some 
other phone to determine whether or not 
the paging signal is sufficiently strong 
to energize all relays within the system. 
Of particular importance is whether or 
not the battery has sufficient voltage to 
energize the paging relay of the tele- 
phone farthest from the phone being 
tested. Therefore, one of the most dis- 
tant phones should be called. Batteries 
can also be checked with a voltmeter to 
judge if they are near the end of their 
life or in a marginal state. There are 
several methods of measuring the avail- 
able battery voltage as noted in the fol- 
lowing section. 

7.2.2e Battery Testing 

Most pager phones are powered by one 
or two 12-volt batteries of the NEDA 923 
or 926 drycell type. These are 12-volt, 
metal-cased batteries that measure 2-3/4 
inches wide, 5-1/4 inches long, and 4-3/8 
inches high. The difference between the 
two types is that the 926 has two screw 
terminals for lead attachment and the 923 
has a two-prong connector system for lead 
attachment. For those phones using a 
24-volt system, two batteries are 
connected in a series. In consideration 
of intrinsic safety, it is common to find 
some means of current limiting, such as a 
50- to 100-ohm resistor and a fuse in se- 
ries with the battery system, to limit 



161 



the maximum current flow. Batteries are 
approaching the end of their useful life 
in a system when the available voltage at 
the terminals has dropped 25% from the 
rated value measured under load condi- 
tions. In a 12-volt system, this is ap- 
proximately 8 to 9 volts; in a 24-volt 
system, it is 16 to 18 volts. 

Measurement of the battery voltage 
can be made by connecting a dc voltmeter 
across the battery terminals, pressing 
the page switch, and reading the battery 
voltage. Measurement of battery voltage 
on the line will not give a true measure 
of the battery condition, because of 
the added voltage drop in the current- 
limiting resistor. 

Remember that it is useless to mea- 
sure the output of a battery not under 
load. Under these conditions, even the 
poorest battery will still maintain its 
rated terminal voltage. 

It is not always practical to carry 
a voltmeter into all sections of a mine, 
and checking a battery requires that the 
phone enclosure be opened. The following 
scheme can minimize such difficulties. 

A voltmeter can be permanently in- 
stalled at some convenient location 
aboveground, such as in a repair or main- 
tenance shop.^ The meter is connected 
across the line so that it continuously 
indicates any dc voltage on the line. A 
listing of voltage readings is made from 
each remote phone at this reference sta- 
tion, when the individual phones are pag- 
ing with new batteries installed. A 
chart is then made of the allowable re- 
duction in voltage for each phone by es- 
timating a 20% to 25% reduction from the 
new battery condition. Reference to this 
chart can give advance warning of the 
approximate condition of each battery 
and will provide guidance for planned 
prelacement. A periodic check can be 
made of each phone by requesting a page 
from each of the phones and maintaining a 

^This could violate MSHA intrinsic 
safety standards; check with MSHA for ap- 
plication details. 



log of the voltage readings. This will 
assist in maintaining an up-to-date sta- 
tus of the battery condition at individ- 
ual phones. This procedure will remain 
valid as long as the phone system is 
configured as it was when the original 
listing was made. Substantial change in 
the phone system could require making a 
new chart. 

7.2.3 Carrier Phones 



CAUTION 

Some of the procedures discussed 
in this manual are undertaken with 
the in-mine trolley wire energized 
and are therefore very hazardous. 
Extreme caution must be exercised 
to avoid accidental electrocution. 
Fuses used in test leads protect only 
the equipment and do not provide any 
protection from shock hazard for the 
operator. Do not attempt any of the 
electrical tests or installations de- 
scribed in this manual unless you are 
qualified for such work and are thor- 
oughly familiar with electrical work 
on trolley wires. 



Each of the carrier phone units 
should be examined for any external phys- 
ical damage. All fixing screws must be 
tight. All connectors and externally ac- 
cessible fuses should be checked for 
proper seating. 

7.2.3a Microphone 

The carrier phone microphone is a 
delicate piece of equipment and is most 
prone to abuse by handling or dropping. 
The microphone should be examined for 
evidence of physical abuse. The action 
of the transmit relay can be observed by 
pressing the transmit button and listen- 
ing for the transmit relay inside the 
transmitter (in units where such a relay 
is used) to produce a sharp click. The 
microphone quality can then be assessed 
by transmitting a test count to a remote 
unit ; the operator of the remote unit 
will judge the quality of the voice he 
receives and report back to the unit 
being tested so that the receiving 



162 



quality of the unit being tested may also 
be assessed. 

7.2.3b Batteries 

Two different types of battery sys- 
tems are used in carrier phones. One is 
a conventional car battery type or wet 
lead-acid cell, and the other is a gelled 
electrolyte battery. Both types should 
be tested once a month to insure a proper 
state of charge, and the electrolyte 
should be checked in wet lead-acid bat- 
teries, if possible. The gelled electro- 
lyte battery is also of a lead-acid con- 
struction; however, its so-called dry 
electrolyte system cannot be changed 
since the cell is sealed to prevent any 
electrolyte loss. Overcharging of either 
of the battery types causes considerable 
electrolyte loss, and both types of bat- 
tery can be ruined if overcharged for a 
long period of time. 



CAUTION 

Electrolyte loss also happens to 
a lesser extent during a normal 
charge cycle and results in the emis- 
sion of hydrogen and oxygen from the 
cells in a ratio which is explosive. 
Slow emission of this hydrogen and 
oxygen gas mixture in the enclosures 
using a gelled electrolytic battery 
can create a hazardous mixture of 
gases inside the units. Some units 
are vented to prevent a pressure 
buildup inside the enclosure, but 
this is insufficient ventilation to 
prevent the possible buildup of a 
hazardous atmosphere inside the box. 
Thus, transceivers using a gelled 
electrolyte battery should only be 
opened in a well-ventilated area 
where there are no possible sources 
of ignition of the hydrogen and oxy- 
gen mixture before it is sufficiently 
diluted by the surrounding air to be- 
come harmless. Wet lead-acid batter- 
ies should be placed in a well- 
ventilated area in the vehicle to 
prevent buildup of pockets of danger- 
ous hydrogen-oxygen mixture. 



7.2.3c Wet Cell Maintenance 

If a car-type wet cell lead-acid 
battery is used, it should be installed 
in a well ventilated area easily accessi- 
ble for routine maintenance. Each week 
the level of the electrolyte in each cell 
should be checked and restored to its 
proper level by the addition of distilled 
water. The electrolyte should read a 
specific gravity of approximately 1.275 
on a battery-testing hydrometer when the 
battery is fully charged. The voltage 
for each cell should be between 2.2 and 
2.4 volts. Since in normal operation the 
battery is under continuous charge, the 
specific gravity and voltage of a battery 
in good condition should be around the 
stated values. Values significantly less 
are sjmiptoms of problems with either the 
battery or the battery charger and should 
be investigated. If electrolyte is lost 
from the battery due to spillage, then 
electrolyte premlxed to the same specific 
gravity should be used to refill the bat- 
tery to its normal level. 

Terminal posts on lead-acid batter- 
ies should be examined and cleaned each 
month. Petroleum jelly may be used to 
coat these posts to prevent corrosion. 
Also, any corrosion of the battery box 
should be scraped clean, and petroleum 
jelly should be applied to prevent any 
further corrosion. 

If a vehicle equipped with a carrier 
phone is to be taken out of service 
for some time, then both battery leads 
should be disconnected to prevent dis- 
charge of the battery while the unit re- 
mains in standby mode. Again, petroleum 
jelly should be applied to the battery 
posts and the terminals to prevent any 
corrosion. 

Any battery found to be in a weak 
condition should be removed for recharg- 
ing and replaced by a fully charged bat- 
tery. If a particular vehicle has re- 
peated battery problems , the battery 
charger in that vehicle should be removed 
for checkout. 



163 



7.2.3d Gelled Electrolyte Battery 

It is not possible to service the 
electrolyte in a gelled electrolyte bat- 
tery since it is sealed at the factory. 
However, these batteries do vent small 
amounts of hydrogen and oxygen during the 
charging process , which will increase to 
larger amounts if the battery is over- 
charged. Normally, the battery should be 
charged by a taper-charge process. This 
means that when the battery is in a dis- 
charged condition, the battery charger 
can apply a comparatively large amount of 
current to build the charge up in the 
battery quickly. However, as the battery 
charge increases , the charging rate 
should decrease. When the battery is al- 
most fully charged, the charging current 
should fall to zero or maintain a very 
small charge. Each of the two types of 
carrier phones using batteries of this 
type have a taper-charge-type battery 
charger built in to maintain the cells at 
a fully charged state, without the hazard 
of overcharging. 

The important parameter to measure 
for proper gelled electrolyte battery 
maintenance is the battery voltage. A 
nominal 12-volt gelled electrolyte bat- 
tery is fully charged when it reads 13.8 
volts across the terminals. This should 
be the voltage reading when the battery 
has been fully charged by the operation 
of its battery charger. Any voltage 
higher than 13.8 is an indication that 
the battery is being overcharged, thereby 
suffering a considerable loss of life due 
to the drying out of the electrolyte. 
This also causes generation of dangerous 
quantities of hydrogen and oxygen gas 
mixtures as the cell vents. If this is 
the case, the battery charger should be 
examined for malfunction. 



discharged and stored in this condition 
without being recharged, the battery may 
develop a condition where it cannot be 
recharged and should be replaced. 

7.2.3e Troubleshooting on the Vehicle 

When an operator reports a malfunc- 
tion carrier phone, initial diagnosis of 
a problem can be carried out using only 
the equipment suggested in table 7-1. 
The repairman may either take his equip- 
ment to the faulty vehicle , or the faulty 
vehicle may be returned to the test and 
maintenance area. 

First, the battery voltage should be 
checked to make sure it has not become 
discharged. If it is found to be good, 
all external fuses in the unit should be 
checked. If a faulty fuse is found, it 
should only be replaced with a fuse of 
the proper rating. If the fuse blows 
again, then the unit is probably faulty. 
It is possible that replacing the blown 
fuse with a new one will cause the unit 
to operate properly since a momentary 
overload could have caused the original 
fuse to blow. 

Sometimes the phone itself can pro- 
vide valuable information on the nature 
of a problem. Use of the carrier phone 
will generally isolate the problem into 
one of the following three categories. 

1. Cannot transmit to others or re- 
ceive from others : 

a. Check the main fuse. 

b. Check the ground connection. 

c. Check all connectors for 
corroded contacts. 



Alternatively, if the carrier phone 
has been left on for an extended time 
without any battery charging from the 
trolley wire, it is possible for the bat- 
tery to become moderately or deeply dis- 
charged. A moderately discharged battery 
can be removed for recharging and gener- 
ally will not suffer any significant 
harm. However, if the battery is deeply 



d. Check all cables for breaks. 

e. Check the battery condition. 

f. If cause cannot be readily 
located, replace with spare unit and 
take the malfunctioning unit in for 
bench maintenance. 



164 



TABLE 7-1. - Suggested test equipment 



Item 



Type 



Use 



Multimeter. 



Various, 



Fuses. 



.do. 



Substitute units. 



Same as used in the 
mine. 



Hydrometer. 



Battery type. 



Distilled water. . 
Petroleum jelly.. 
Battery charger. . 



Any, 



Any applicable (for 
wet battery) or 
special battery 
charger for gelled 
electrolyte 
battery. 



Meter can be used for measuring voltages in 
and around the unit, power consumption, 
power output, and fuse checking. In order 
to give useful results for transmitter power 
output measurements, the meter should be 
capable of operating with frequencies of at 
least 100 kHz. 

Fuses provide an intentionally weakened part 
of an electric circuit, and thereby act as a 
safety valve in the event of dangerous over- 
loads. This protects both personnel and 
equipment from potential fire hazards due to 
overheating of the carrier phone. 

A blown fuse generally indicates that some 
part of the circuit of the carrier phone has 
become defective. Occasionally a temporary 
external overload condition can cause a fuse 
to blow; hence it is a useful practice to 
change a blown fuse one time to see if the 
unit can be brought back into service. 
Should the fuse blow again, then a more 
detailed trouble-shooting process should be 
attempted. 

Each carrier phone consists of a number of 
different units interconnected by cables. 
To facilitate troubleshooting on the vehi- 
cle, a fully operational spare set of the 
type used in the mine should be maintained 
so that initial trouble-shooting can be per- 
formed by substitution of the individual 
units. 

The hydrometer measures the charge-discharge 
condition of the battery electrolyte. 

A battery with a low level of electrolyte 
will require an addition of distilled water. 

Coating the battery terminals with petroleum 
jelly aids in preventing corrosion. 

The battery charger is used to recharge bat- 
teries that have become discharged. 



NOTE. — Insure equipment is suitable for 
desired application. 



165 



2. Can hear others but cannot ap- 
parently transmit: 

a. Check cables and connectors 
(especially the microphone for cor- 
roded contacts or breaks). 

b. Replace the microphone. 

c. If neither the above is at 
fault, the problem is probably in 
the transmitter; take the malfunc- 
tioning unit in for bench 
maintenance. 

3. Cannot hear others but they can 
hear your transmission: 

a. Check the volume control 
setting. 

b. Check the cables and connec- 
tors for breaks and corrosion. 

c. Replace the speaker assembly 
by substitution. 

d. Check the squelch setting. 

e. If none of these measures 
solve the problem, it is probably 
in the receiver; replace the trans- 
ceiver with a spare unit and take 
the malfunctioning unit in for bench 
maintenance. 

If all these steps fail to make the 
unit operational, then repair by substi- 
tution is usually the quickest way of 
getting the unit into operation again. 
Substitution should be in the order of 
items considered to be more or less vul- 
nerable. Unless it is obvious which unit 
is faulty, the process should be carried 
out in the following order: 

1. Change the microphone assembly 
and test for normal operation. 

2. Change the transceiver assembly 
and check for normal operation. 

3. Change the loudspeaker unit and 
check for normal operation. 



4. Where relevant, change the bat- 
tery charger box and check for normal 
operation. 

When the faulty unit has been iso- 
lated and replaced, it should be returned 
to the repair area for a more detailed 
examination, including an overall per- 
formance checkout after the fault has 
been isolated and repaired. 



CAUTION 

The following procedure is un- 
dertaken with the trolley wire ener- 
gized; therefore, it is extremely 
hazardous. Extreme caution must be 
exercised to avoid potentially lethal 
shock. Only personnel thoroughly 
familiar with electrical work on 
trolley wires should conduct the test 
procedures. Equipment used must be 
appropriate for this application. 



On 300-volt systems, a test can be 
made of the transmitter power output onto 
the trolley line. A simple method of 
measurement in the field makes use of the 
multimeter with the range selector switch 
set to the 50-volt-ac scale. The black 
meter lead should be plugged into the 
column (-) terminal of the meter and the 
free end connected to the ground. The 
red lead must be plugged into the meter 
output jack and connected to the trolley 
wire. The trolley pole must be in con- 
tact with the wire. A reading of 15- 
volts or more when the transmitter is 
keyed indicates normal operation provided 
the test is made at least 200 feet from 
the nearest power rectifier that supplies 
the trolley wire. This test cannot be 
performed on 600-volt dc systems since 
this voltage will overstress some compo- 
nents inside the multimeter. In this 
case, the unit should be returned to the 
repair shop for a standard bench test. 
It should be noted that the meter will 
respond to ripple present on the trolley 
wire; thus a base reading of up to 10 
volts will be shown even with the trans- 
mitter off. 



166 



7.2.3f Mapping Signal Levels 

The maintenance of trolley carrier 
phone systems requires not only the main- 
tenance of the equipment involved, but 
the maintenance of the transmission line 
(trolley wire-rail) used to transmit the 
signals (refer to paragraph 5.3.1). Evi- 
dence accumulated over the years indi- 
cates that this signal path is subject to 
many loads that impede the propagation of 
carrier signals. 

One of the most useful ways of de- 
termining the state of the overall 
transmission system is to map the signal 
and noise strengths at various points 
throughout the mine. Such mapping re- 
quires a tuned signal-measuring device. 

The mapping is preferably carried 
out by measuring the signal produced 
by the dispatcher's transmitter at vari- 
ous points along the rail haulage system 
where vehicles operate. A satisfactory 
way of conducting the measurements is 
to place a suitable tuned voltmeter 
aboard a mine vehicle (such as a jeep) , 
and at appropriate places along the rail 
haulage — for example, at 2,000- or 3,000- 
foot intervals — measure the received dis- 
patcher's signal and background noise. 
These values should be noted on a mine 
map for future reference as the mine ex- 
pands, or as carrier phone problems oc- 
cur. Except under extremely unusual con- 
ditions, the signal-strength map produced 
in this manner will also indicate the 
level of signal that a vehicle transmit- 
ter at the measuring position would pro- 
duce at the dispatcher's place. A por- 
tion of a mine map with signal and noise 
readings is shown in figure 7-1. 

The equipment for making such a 
signal-strength map must be battery oper- 
ated, easily portable, and easy to use 
and read. Two such units commercially 
available are shown in figure 7-2. These 
tuned voltmeters are general-purpose, 
battery-operated instruments appropriate 













ffi 



•'i'^ 












rzprs 














FIGURE 7-1. - Example of signal level map. 

for many tasks other than the mapping of 
trolley carrier signal levels. For this 
reason careful attention must be paid to 
the tuning of the instrument to the pre- 
cise frequency, attenuator settings, and 
meter indications. Table 7-2 gives spec- 
ifications for these tuned voltmeters. 

The simple straightforward procedure 
of measuring the dispatcher's signal lev- 
el from a jeep or vehicle moving about 
the mine can best be accomplished by con- 
necting the trolley wire voltage on board 
the vehicle to the input of the tuned 
voltmeter. Because of the hazards asso- 
ciated with the high voltage of the trol- 
ley wire, either voltmeter has to be 
properly Isolated so that personnel oper- 
ating the instrument are not subjected to 
this voltage through error in operation. 
Therefore, it is important that a capaci- 
tor and a fuse be connected in the series 
with the instrument to insure that the 
potentially lethal voltage of the trolley 
wire does not inadvertently reach an 
operator. Figure 7-3 shows a possible 
way of connecting the instruments. 



TABLE 7-2. - Key specifications of tuned voltmeters 



167 



Specification 

Frequency range kHz , 

Accuracy: 

Frequency kHz. 

Level dB. 

Selectivity (standard 250 Hz), Hz: 

3-dB bandwidth 

35-dB bandwidth , 

60-dB bandwidth , 

Ranges (full scale) , 

Intermediate frequencies, kHz: 

1st , 

2nd 

Power requirements 

Voltage , 

Battery life (zinc carbon) ... .hours, 

Temperature range °C. 

Dimensions, inches: 

Width 

Height 

Depth , 

Weight pounds. 

NAp Not applicable. 



Sierra 


127C 






Ry 


=om 3115 






2-350 






3-200 






±1 
±1 






±1 
±1 




1 mV 


250 

600 

1,000 

to 10 V 


-37 
mV 


to 
to 


1,000 
NAp 

4,000 
+13 dB (3.7 
1.65 V) 


6 zinc-carbon 
rechargeable 


1,305 

330 

or 7 NiCd 

D-size cells 

9 (nominal) 

100 

-10-50 




5 


NAp 

NAp 

2 gel cells 

Globe 610 

12 

(continuous) 

-10-55 






12 
7-1/2 
7-1/2 

15 






7-1/4 

5-1/4 

7-3/4 

6 



WARNING 

Disconnect the instrument when 
the vehicle is moving. Transients 
from the vehicle motor can cause 
damage. 



CAUTION 

The following procedure is un- 
dertaken with the trolley wire ener- 
gized; therefore, it is extremely 
hazardous. Extreme caution must be 
exercised to avoid potentially lethal 
shock. The fuses used in the test 
leads serve only to protect equipment 
and do not in any way reduce the 
shock hazard to personnel. Only per- 
sonnel thoroughly familiar with elec- 
trical work on trolley wires should 
conduct the test procedures. Equip- 
ment used must be appropriate for 
this application. 



To make a measurement , the vehicle 
is moved to the desired location in the 
mine and stopped. The operator then asks 



the dispatcher to "key" his transmitter 
for a 5-second transmission of unmodu- 
lated carrier. The response on the indi- 
cating meter is noted, together with any 
attenuator setting, so that an absolute 
value of voltage (in volts rms) can be 
noted on the corresponding position on 
the mine map. It may be necessary when 
starting measurements to switch the range 
knobs of the instruments to make sure 
that the instrument's response is on 
scale rather than high and off the scale. 
In this event, perhaps two transmissions 
will be required before on-scale readings 
are obtained. After the transmission 
from the dispatcher is recorded, the sen- 
sitivity of the instrument should be in- 
creased and the noise level at the par- 
ticular position noted again in volts or 
millivolts rms. 

The signal-level map will reveal re- 
gions of the mine where the dispatcher 
signals are weak which may cause diffi- 
culties in carrier communication. The 
mine map will also reveal regions where 



168 




V ^ -• 



RYCOM 3115 




FIGURE 7-2. - Tuned voltmeters. 



TROLLEYWIRE 




SIGNAL LEAD 

GROUND TO 
NEAREST NUT 
OR BOLT HEAD 



FIGURE 7-3. " Instrument connections. 



excessive noise is the main cause of poor 
communications. In this event, it is im- 
portant to locate the source of the of- 
fending noise and to take measures to al- 
leviate the problem. 

The signal-level map will also be 
extremely useful should carrier communi- 
cations deteriorate with time, with the 
installation of new equipment, or with 
the advancement of the mine. Reference 
can be made back to the original signal 
levels to determine if and why communica- 
tions have been degraded. 

7 . 3 Summary 

Good preventive maintenance and pe- 
riodic inspection practices are the key 
to successfully maintaining any communi- 
cation system. Common problems that can 
affect communications include: 

Corrosion or conductive dust on bat- 
tery terminals. 

Cable abrasion and line breaks. 

Corrosion on switch contacts or in 
cable splices. 

Weak batteries. 

Blown fuses. 

Weak or broken springs on spring- 
loaded connectors. 

Poor splicing techniques. 

In addition to problems that develop 
owing to normal system usage and environ- 
mental conditions, trolley carrier sys- 
tems may be affected by characteristics 
of the trolley wire and rail itself. 
Poor signal strength may result because 
of bringing loads across the trolley 
wire-rail or high signal attenuation 
rates in the trolley wire. Methods of 
compensating for the effect of bridging 
across the wire-rail are given in section 
5.3.1. 



BIBLIOGRAPHY 



169 



1. Long, R. G. Guidelines for Instal- 
lation, Maintenance and Inspection of 
Mine Telephone Systems. BuMines OFR 116- 
78, June 1975, 53 pp.; NTIS PB 287 641. 

2. Long, R. G. , R. L. Chufo, and R. A. 
Watson. Technical Guidelines for In- 
stalling, Maintaining, and Inspecting 



Underground Telephone 
Handbook, 1978, 44 pp. 



Systems. BuMines 



3. Spencer, R. H. , P. O'Brien, and 
D. Jeffreys. Guidelines for Trolley Car- 
rier Phone Systems. BuMines OFR 150-77, 
March 1977, 170 pp.; NTIS PB 273 479. 



170 



APPENDIX A. —COMMUNICATION SYSTEM EXAMPLESi 



A.l INTRODUCTION 

Because no two mines are identical, 
there is no "one best system" that can be 
defined to meet the requirements of all 
mines. The following examples of system 
installations are presented to indicate 
how some mines have adapted available 
equipment to meet their particular 
requirements. Selection of examples were 
based on the goal of obtaining the widest 
possible range and cross section of the 
following characteristics: 

Type of mine 

Age and size of mine 

Electrical power usage 

Haulage methods 

Existing communications 

Usefulness of present communications 

Examples A through F are of coal 
mines utilizing various combinations of 
magneto, pager, and conventional trolley- 
carrier-type phone systems. Example G is 
of a magnetite ore block-caving operation 
where a radiating cable and radio system 
is used. Example H indicates how a dial- 
page phone system can be utilized in a 
coal mine. Example I presents a multi- 
channel (multiplexed carrier) system 
presently in use in a deep metal mine. 

A. 2 MINE A 

Mine Description 

Mine A is part of a connected four- 
mine complex. This particular mine is 
approximately 20 years old, and although 
there are some new working sections, the 
major coal extraction is from retreat 
mining where pillars are being pulled. 
Personnel entry is achieved through a 

^Use of company names is for identifi- 
cation purposes only and does not imply 
endorsement by the Bureau of Mines. 



vertical shaft approximately 545 feet 
deep. Coal is removed from the face area 
by shuttle cars and placed in a set of 
six tracked haulage cars. When full, 
the sets of cars are combined into trains 
and brought to the surface through a 
slope entry. Average coal production is 
4,000 to 5,000 tons per day for 240 work- 
ing days, setting yearly production at 
approximately 1 million tons. 

The mine size is currently 2.4 miles 
north and south by 3.9 miles east and 
west with overburden from 545 feet to 
1,000 feet. All tunnels and haulageways 
are typically 6.5 feet high by 14 to 
15 feet wide. An average working section 
is 425 feet by 300 feet long, and 10-foot 
roof bolts are used. The mine has only 
one borehole, which is used to supply the 
mine with water. 

Currently the mine has six working 
sections of which five are worked every 
shift. The shifts run from 8 a.m. to 
4 p.m. , 4 p.m. to 12 p.m. , and 12 p.m. to 
8. a.m. A typical working section cycle 
starts with the continuous miner cutting 
coal and filling a shuttle car. When the 
shuttle car is full, the driver moves the 
coal load to the tracks and transfers the 
coal to one of the six haulage cars posi- 
tioned on the side track. The shuttle 
car then returns to the continuous miner 
to repeat the cycle. Excluding mechani- 
cal trouble, the continuous miner will 
cut a block of coal 5 feet high, 15 feet 
wide, and 16 feet long in 1 hour, and a 
section can mine five blocks this size in 
an 8-hour shift. The mine typically has 
100 men underground per shift. 

Mine Equipment and Power 

The prime power for the mine is 
550 volts brought in on a feeder cable. 
In the mine the trolley wire is run par- 
allel to the feeder cable. At the work- 
ing section the continuous miner, shuttle 
cars, and car pull are run off the 550- 
volt-dc trolley line fed at nip stations. 
Compressed air is used to run the roof- 
bolting machine. 



171 



The equipment at each working sec- 
tion includes one continuous miner, two 
shuttle cars, one roof -bolting machine, 
and one car pull. Other equipment 
includes 3 bottom-loading machines, 

2 minor-type cutting machines, and 
12 pumps. 

The tracked vehicles include 3 dual 
locomotives or tandems, 24 Jeeps, and 

3 portal buses. 

Present Mine Communications 

The present communication consists 
of a carrier phone system and a magneto- 
phone system. All vehicles are equipped 
with FEMCO carrier phones, and all active 
working sections, along with selected 
underground positions, have Western Elec- 
tric magnetophones. 

Telephone System 

The heart of this mine's communica- 
tion system is a central dispatcher 
located at the bottom of the main shaft. 

Eight party-line magnetophone cir- 
cuits terminate at a simple switchboard 
in the dispatcher's office. Each of 
these 8 circuits has several of the 
41 telephones wired in parallel. Calls 
between circuits must be made through the 
dispatcher and his or her switchboard, 
whereas calls within a circuit need not. 
The dispatcher can connect any two phone 
circuits together and can make two of 
these connections, generating two inde- 
pendent phone circuits for two-channel 
operation. 

Since this magnetophone system oper- 
ates with a bell ringer rather than a 
loudspeaker, the rings are coded to indi- 
cate certain places or individuals. The 
dispatcher communicates through a single 
headset, and selection of either the mine 
phone or the carrier phone is made using 
a two-position switch. Other switches 
connect and disconnect the various mine 
telephone circuits. 

This dispatcher controls all vehicle 
traffic and serves as a telephone 
operator. Operator duties include 



answering phone calls, switching phone 
circuits, personnel calling and location, 
and taking and relaying messages. 

Because the dispatcher is more 
likely to contact a working section 
through the motor and an associated car- 
rier phone in that section, the mine 
phone is used relatively little conqjared 
with the carrier phone. 

Based on the observed traffic den- 
sity and on the number of phones in the 
system, the probability of a busy signal 
on the magnetophone system is 5%. 

Trolley Carrier Phone System 

Vehicle-mounted carrier phone usage 
during a typical shift is shown in figure 
A-1. 

During a first shift survey, there 
were 182 dispatching calls, 20 calls 
relating to personnel location, and 58 
calls relating to placing empty and 
loaded cars. 

Communications Requirements — ^Users ' 
Viewpoint 

Evidence of this mine's interest in 
communications is shown by the expression 
of one foreman that "their production 
would be cut in half if they lost either 
telephone or carrier phone communica- 
tions." An important comminications. 
requirement as defined by the management 
of this mine concerned safety. They 
strongly felt that a secure channel was 
needed where only the persons calling and 
called could hear the conversation. 
There are two reasons for this: First, 
anyone seeking aid for an injured miner 
tends to belittle the seriousness of the 
injury because he knows that friends and 
relatives of the injured miner, and those 
just curious, will be listening to the 
conversation. The problem is not unique 
to this mine. Secondly, the phones of 
these eavesdroppers load the line to the 
extent that the emergency conmiuni cat ions 
are impaired. 

Based on this realistic situation, a 
basic communications requirement is a 



172 



£ 
d 

UJ 

CO 



< 

O 

I- 




8-9 


9-10 


10-11 


11 a.m.- 


Noon- 


1-2 


2-3 


3-4 


a.m. 


a.m. 


a.m. 


noon 


1 p.m. 


p.m. 


p.m. 


p.m 



FIGURE A-1. = Vehicle-mounted carrier phone use in typical shift. 



private line, selective calling channel 
over which the person attending an 
injured person can privately call, at his 
or her discretion, the mine foreman, the 
dispatcher, the safety foreman, or the 
nearest hospital or ambulance service. 
Note that a conventional private dial 
system meets this requirement. Mine C 
(described in section A-4) has a dial 
phone adjacent to each pager phone. This 
met the need for a secure channel 
for both management and emergency 
communications . 

Communications Requirements — Based 
on Survey Analysis 

Although the personnel interviewed 
felt the quality of their communications 
was adequate, analysis indicates that 
excessive noise and distortion were pres- 
ent. Therefore, a requirement that 
applies to this mine as well as to all 
communication systems is that of reason- 
able signal-to-noise ratio for good 
intelligibility. 



The fact that the chance of getting 
a busy signal is 5% is proof that addi- 
tional channel capacity is needed. Add- 
ing additional channels to a wired system 
appears to be an acceptable solution 
since these extra channels will minimize 
the telephone duties the dispatcher now 
performs and will eliminate the communi- 
cation system blocking problem. Calcula- 
tions indicate that a minimum of five 
communication channels are needed for 
this mine. Furthermore, making one of 
the five channels a private line will 
fulfill the requirement for private 
communications . 

Also, from observing the mine opera- 
tion and talking to various personnel it 
appears that section foremen, like the 
foremen in most industrial operations, 
are overworked, and yet are the key to 
improving productivity. Therefore, wire- 
less communication is needed for at least 
the section foremen along with vari- 
ous other supervisors and maintenance 
personnel. 



173 



From this brief analysis of the mine 
and its current communications, the fol- 
lowing is a list of minimum communication 
requirements for Mine A. 

a. Reasonable communication channel 
signal-to-noise level. 

b. At least five independent voice 
channels. 

c. At least one secure voice chan- 
nel, which may be included in the five 
voice channels. 

d. Some form of wireless communica- 
tion to select individuals on the working 
section or roving in haulageways. 

A. 3 MINE B 

Mine Description 

Mine B consists of adjacent (No. 1 
and No. 2) low-coal mines. The No. 1 
Mine employs longwall and continuous min- 
ing. The No. 2 Mine employs conventional 
and continuous mining and is preparing 
for its first longwall operation. Both 
mines employ belt coal haulage to closely 
located drift entrances. Men and sup- 
plies enter the No. 1 Mine by a 400-foot 
shaft remotely located from the No. 2 
Mine drift entrance. From the two mines, 
8,000 tons of coal per day are mined by 
about 600 union men under the supervision 
of about 60 officials. 

The No. 1 and No. 2 Mines each cur- 
rently employ one longwall mining unit 
and conventional working sections of Lee 
Norse continuous miners. For the No. 1 
Mine's longwall mining, coal is moved by 
an armored face conveyor to a stage 
loader at one end of the longwall, to an 
extendable belt, and finally to a con- 
ventional belt. 

In addition to longwall mining, the 
No. 2 Mine employs a full-dimension unit, 
a conventional mining unit, and continu- 
ous miners for seven working sections per 
shift. 

The equipment used in conventional 
mining consists of a cutting machine, a 



loader, and two shuttle cars. With a 
full-dimension system, the shuttle cars 
are replaced by an extendable belt. 

Coal is brought out of the two mines 
by conveyor belt, and men and supplies 
are moved by track. The coal is moved by 
belt from the two mines to a screening 
house having 1,250 tons of storage capac- 
ity. To cope with slacks and overflows, 
coal can be automatically diverted to a 
12,000-ton-capacity storage pile. 

Ac power is brought into the mines 
at 12,470 volts. Two rectifiers are 
positioned at every 6,000 feet of track, 
each with a capability of 300 kW, to 
supply 300-volt-dc power to the trolley 
wires and their feeders. In addition to 
supplying locomotives with power, the 
trolley lines supply power, at nip points 
along the line, for the operation of the 
300-volt-dc shuttle cars. Where needed, 
the 12,470 volts ac is transformed to 
600 volts to provide ac power for rock 
dusters, conveyor belt drives, miners, 
roof bolters, and belt feeders. 

Present Mine Communications 



The equipment used in each of the 
two mines includes paging-type party line 
telephones, trolley carrier phones, the 
fire sensor tape recording that would 
automatically be patched into the phone 
system should there be a belt fire, and 
the fan sensors that utilize the phone 
lines. 

The No. 1 Mine's communications sys- 
tems are independent of the No. 2 Mine's, 
but generally of the same size and equip- 
ment types. The No. 2 Mine's chief 
electrician's office has a No. 1 Mine 
phone, as does the No. 2 Mine's foreman's 
office. 

Telephone System 

Since both mines have similar com- 
munication equipment, only the No. 2 Mine 
will be described. The No. 2 Mine has 
31 underground loudspeaking telephones. 
The underground phones are all in a 
single network. The following seven sur- 
face phones are also in this network: 



174 



Outside mechanics shanty — 1 

Outside shop — 1 

Auditorium — 1 

Chief electrician — 1 

Cleaning plant — 2 

Double breaker switchhouse — 1 

The paging phones used in these 
mines use 6 volts for normal phone use 
and 22.5 volts during paging. With these 
phones, pressing the paging button at any 
station permits the operator to broadcast 
through the loudspeakers on the remaining 
37 telephones. On releasing the paging 
button the operator can converse with 
anyone who picks up the handset on any 
other phone. 

Tape recordings were taken of both 
the No. 1 and the No. 2 Mine's party line 
pager phone system and the No. 1 Mine's 
carrier phone system. Analysis of these 
recordings revealed that for the No. 2 
Mine, based on hour intervals, the most 
the system is used is about 50% of the 
time, between 9 and 10 a.m. But, based 
on 15-minute intervals, the phones are 
used nearly 90% of the time around 3:30 
in the afternoon. 

This heavy usage occurs during the 
last hour of the shift when section fore- 
men are making their end-of-shift reports 
on production status, supplies on hand, 
supply requests, and maintenance work 
requests. The fact that the phone system 
is used 90% of the time signifies that 
other calls that could improve production 
efficiency must either be delayed or not 
made at all. 

Carrier Phone System 

A second means of voice communica- 
tions is the carrier phone system that 
uses the dc trolley wire as a carrier of 
88-kHz (No. 2 Mine) and 100-kHz (No. 1 
Mine) FM. In each mine five carrier 
phones are used: One as a base station 



at the inside mechanics shanty, 
utility jeeps, and two on motors. 



two on 



One shortcoming of the present car- 
rier system is that there is no way for 
personnel with carrier phones to communi- 
cate with working sections. Mine per- 
sonnel would like some way of patching 
the carrier and pager phone systems 
together. 

Longwall Communications 

Five permissible loudspeaking tele- 
phones are spaced at 125-foot intervals 
along the 500-foot longwall system. 
These five phones are connected together 
to form an independent communications 
system. Near the phones at either end of 
the longwall system are phones of the 
overall telephone network. Though not 
interconnected, the two phone networks 
are physically close to each other. 

The five phones are identical to 
those of the main telephone system except 
that the paging mode is permanently wired 
into all five phones as a safety measure. 
Anything said into any one of the five 
handsets will be broadcast over all five 
loudspeakers, thus alerting all nearby 
personnel of activity on the longwall 
section. 

Belt Maintenance Communications 

Along the belt lines (every 
2,000 feet) and at the belt heads are 
located phones of the telephone communi- 
cations system. The belt heads are the 
only spots where belt mechanism fires are 
likely to occur. 

Belt Fire Alarm System 

Although these mines have never had 
a belt fire, their fire alarm system is 
better than required by law. A tape 
player is positioned underground and when 
activated will broadcast a warning over 
all telephone and carrier phone loud- 
speakers. The recorded message warns 
all personnel of the alarm condition, 
specifies the location of the tripped 



175 



alarms , and advises personnel of safety 
precautions to be taken. 

Fan-Stop Alarm 

In the event a fan stops, provision 
is made for utilizing the phone systems 
to insure that proper action is taken. 
At the No. 1 Mine, where there is a phone 
at the fan site and where personnel are 
within earshot of an audible alarm, the 
person responding to the alarm can use 
the normal telephone system in seeking 
help. The fans for the No. 2 Mine are 
remote from any mine personnel so the 
alarm is automatically sent over a com- 
mercial phone line to No. 2 Mine's 
lamp house. 

Communications Requirements — User's 
Viewpoint 

Through interviews and discussions 
with those who use, plan, and maintain 
the communications systems , communica- 
tions requirements were determined that 
would aid production at these mines. 
These requirements dealt directly with 
mine operations not using a dispatcher, 
with operations where coal haulage is by 
belt only, and with operations involving 
low-coal and longwall mining. 

The first suggestion made by mine 
personnel was that they needed someone to 
perform the communications and informa- 
tion center tasks often performed by the 
dispatcher in other mines. Presently 
they have no way of relaying messages 
between, or interlinking, the independent 
telephone and carrier phone systems. 
They also would like someone to monitor 
belt line sensors, from a center, in 
order to coordinate troubleshooting, 
maintenance, and repair of all belt 
lines. Thus, a requirement for a com- 
munications center operator (communica- 
tions coordinator) would resolve the two 
immediate problems as well as many 
others. 

Low coal and longwall mining com- 
bine in determining a requirement for 
fixed communications terminals to be 
close to all classes of foremen, and a 



requirement for personal hands-off- 
operation radios of insignificant weight 
and bulk. Coordinating the operation and 
and repair of a 500-foot longwall miner 
is difficult, especially in low coal. 

Mine personnel felt that having 
nothing would be better than having a 
simple radio pager where a section fore- 
man might have to crawl 700 feet to the 
nearest phone to find out that it really 
wasn't that important. If the section 
foremen are given anything for mobile 
coimnuni cat ions , it must be small, light, 
and two-way. In this mine they would 
like the section foremen to be able to 
easily contact a general assistant fore- 
man for supplies and repairs. The need 
for small portable two-way communications 
is shown by the case where someone at the 
mine, on his own initiative, tried some 
two-way units he had borrowed from a 
local hospital. 

Communication Requirements — Based 
on Survey Analysis 

An analysis of both the No. 1 and 
the No. 2 Mine survey indicates that the 
communications systems noise levels were 
unacceptably high and that communications 
capability is on the verge of becoming 
unacceptable. Improved communication and 
improved mining operation would result 
merely by improving the signal-to-noise 
ratio of the present communications 
systems. 

Since the current phone traffic 
makes the chance of getting a busy signal 
between 350 and 450 times greater than 
most industries find acceptable, addi- 
tional channel capacity is needed to 
reduce the chance of blocking to the 1% 
level. Although blocking is still 
10 times greater than industrial stan- 
dards, it appears to be a reasonable 
selection for mine communications. 

Also, from observing the mine opera- 
tion and talking to various personnel, it 
appears that section foremen and select 
longwall personnel need some form of 
two-way wireless communication of minimum 
size and weight. All personnel expressed 



176 



a negative attitude toward any one-way 
type of communication. 

Furthermore, the mine personnel felt 
they needed a location where the daily 
production activity could be monitored. 
This location can evolve into a communi- 
cation center, since as the mine expands 
and more vehicles are equipped with 
trolley carrier phones, a combination 
dispatcher, call monitor, and production 
monitor can be financially justified. 

Specifically for these mines, the 
following represents a minimum for future 
communication requirements: 

a. Reasonable communication channel 
signal-to-noise ratio. 

b. At least five independent voice 
channels to replace the present one 
channel. 



transferred by shuttle cars to a set of 
6 haulage cars; 2 of these combinations 
of 6 cars are attached to make a 12-car, 
train, which removes the coal from the 
mine. 

The B seam is currently 1.7 miles 
north and south by 3.2 miles east and 
west; the C seam is 0.35 mile north and 
south by 0.45 mile east and west. The 
overburden ranges from at the slope 
entry to 2,000 feet. All tunnels are 
typically 18 feet wide. The average 
C seam tunnel is 9 feet high, and the 
typical B seam height is 15 feet. Since 
the B seam has coal 22 feet thick in some 
places, the top level is mined first; 
they return to mine the bottom coal for 
maximum yield. Roof bolt length is typi- 
cally 6 feet with variations from 4 to 
12 feet. These bolts are positioned 
on 5-foot centers in the B seam and on 
4-foot centers in the C seam. 



c. Small, lightweight wireless two- 
way communicator units for foremen and 
select personnel. 

d. A communication center. 
A. 4 Mine C 

Mine Description 

Mine C has been operational since 
1903 with production originally estimated 
for 100 years. Coal is being mined in 
the B and C seams, and there is 36 to 
60 feet of vertical displacement between 
these seams. The mine employs continuous 
mining techniques, and personnel enter 
the mine through a slope entry. Coal 
production is approximately 1 million 
tons per year and is removed by a com- 
bination of belt and haulage cars. The 
B seam has two active working sections, 
and each section transfers coal from 
shuttle cars to a small feeder belt. A 
longer mother belt then takes the coal 
to a main loader head. This loader head 
has the capacity for 18 cars; when 12 
cars are filled, these cars are assembled 
into a 12-car train 240 feet in length 
for main line haulage. The C seam has 
only 1 working section, and coal is 



The mine has one borehole into the 
B seam, which was used at one time for a 
phone line and another time for pumping 
water out of the mine. A second borehole 
in the C seam is used to pump methane out 
of the mine. 

Currently the mine has two coal- 
producing sections in the B seam and one 
coalproducing section in the C seam. 
Furthermore, the B seam has one large 
cleanup section and two smaller cleanup 
or rehabilitation sections. A typical 
working section is 320 feet square. The 
mine has two production shifts and one 
maintenance shift, and they run from 
8 a.m. to 4 p.m. , 4 p.m. to 12 p.m. , and 
12 p.m. to 8 a.m. The mine has 71 men 
underground for the first production 
shift, 55 men underground for the second 
production shift, and 45 men underground 
for the last or maintenance shift. 

Mine personnel typically require 
20 minutes to get from the portal to 
their working sections, and they take 
30 minutes for lunch some time between 
11 a.m. and 1 p.m. These lunch periods 
are staggered between working sections. 
On the B seam the work cycle starts with 
the continuous miner cutting coal and 



177 



filling a shuttle car. When the shuttle 
car is full, the driver transfers the 
coal load to one of the 36-inch 550-fpm 
belts run to the section. The belts then 
remove the coal to the south loader head, 
where it is loaded into cars that will 
make up the main line haulage train. The 
shuttle car round trip takes approxi- 
mately 7 minutes to complete a work cycle 
on the B seam. Except for the shuttle 
cars dumping directly into haulage cars, 
the C seam has the same type of work 
cycle. Furthermore, the C seam has no 
belt haulage and uses tracked haulage for 
coal removal. 

Mine Equipment and Power 

Three surface substations convert 
44,000 volts three-phase to 4,160 volts 
three-phase, the 4,160 volts is carried 
underground to various 440-volt-ac and 
275- to 300-volt-dc power stations. 

At the working sections, the shuttle 
cars are powered either by 440 volts ac 
or 275 to 300 volts dc; the continuous 
miner is powered by 440 volts ac, and the 
roof bolting machines are powered by 275 
to 300 volts dc. The dc voltage can be 
obtained by either a trolley nip point or 
an ac-dc load center. 

All trolleys are powered by a 275- 
to 300-volt-dc trolley line, which is a 
common ring bus fed by 300-kW recti- 
fiers and two 500-kW rectifiers. The 
quantity and type of trolleys or vehicles 
follow: 

Man-trip cars — 7 

Mechanics' jeeps — 2 

27-ton motor — 6 

13-ton motor — 2 • 

Present Mine Communications 

This mine utilizes a combination of 
loudspeaking paging phones, dial tele- 
phones, and 88-kHz vehicle-mounted car- 
rier phones. The carrier phone system is 
tied electrically to the loudspeaking 



paging phones by a trolley coupler. This 
type application should not be used with 
intrinsically safe phones. To improve 
communication coverage, auxiliary speak- 
ers are sometimes used with the loud- 
speaking paging phones. The following 
tabulation shows the number of phones and 
their general location: 



Phone type 


B 


C 


Sur- 


Stor- 


Vehi- 




seam 


seam 


face 


age 


cle 


Dial 












telephone. . . 


12 


1 


16 








Loudspeaking 












pager phone. 


5 


1 


1 


1 





Carrier phone 


1 





1 





17 



In general, vehicle operators and 
supervisory personnel use the trolley 
carrier phones, and mine section foremen, 
maintenance personnel, and supervisory 
personnel use the dial telephones and 
pager phones. 

Dial Telephone System 

The mine has purchased from the tel- 
ephone company dial telephones, telephone 
environmental enclosures, associated 
PABX, and 25-pair telephone cable with 
wire size No. 19. All underground tele- 
phone equipment and wire was installed by 
mine personnel and has been in service 
for the last 10 years. Standard dial 
telephones are mounted in environmental 
enclosures. 

The biggest problem the mine has had 
with the dial telephones was dust and 
moisture getting into the dial mechanism. 
This was understandable since it was 
found that the majority of the under- 
ground telephone enclosures had been left 
wide open and were liberally rock dusted. 
Of the eight underground telephones 
checked, seven were in good working order 
and one had rock dust in its dial mechan- 
ism contacts. Another problem, not 
related directly to the telephone equip- 
ment, was that of acoustic noise from 
mine machinery. For example, a telephone 
is required near the 3d south loader 
head, and mine personnel find comminica- 
tion difficult when the loader head is 
in operation. Mine officials have 



178 



considered building a telephone enclosure 
that will shield this telephone from 
external acoustic noise. 

Other than leaving the door to the 
environmental enclosures open, the mine 
personnel appeared to operate the tele- 
phone system properly. The telephone was 
mostly used to call from underground sta- 
tions to surface stations or call out of 
the mine. Most calls were for supplies, 
maintenance, and location of personnel. 
This telephone system was also used as a 
backup system when commini cat ions were 
bad. For example, once personnel were 
contacted using the trolley carrier phone 
or pager phone and extended conversation 
was needed, the person would be 
instructed to go to the nearest dial 
extension telephone and reestablish con- 
tact to complete the communication. 

Over the history of the mine, only 
one major emergency has occurred, a 
destructive fire. This fire destroyed 
the telephone cable, and the underground 
dial telephone system could not be used 
for emergency personnel evacuation. This 
points out the basic weakness that tele- 
phone systems without loopback paths have 
during a real disaster situation. 

Loudspeaking Paging Phone — Carrier Phone 
System 

Carrier phones installed at the mine 
include six 10-year-old units and thir- 
teen 14-month-old units, all with 88-kHz 
center frequency. The mine personnel 
plan to replace the older carrier models 
with new models in the near future. The 
carrier phone to paging phone coupler ( an 
application that cannot be used with an 
intrinsically safe phone system ) is of 
standard manufacture, and auxiliary 
speakers are used with the paging phones 
where the need arises. The loudspeaking 
paging phones communicate through a pair 
of wires from the 25-pair telephone 
cable, and the carrier phones use the dc 
trolley wires for their signal paths. 

With the exception of service prob- 
lems with the older carrier phones, all 
carrier and paging phones are of good 



quality, are holding up well, and are 
apparently being properly used by working 
personnel. However, there is a problem 
associated with the carrier phones com- 
municating from certain dtad zones in the 
mine to the surface. Another problem 
with the carrier phones was that the 
battery had to be serviced every 30 days 
and mine personnel said this was excess- 
ive. Also, the mine officials indicated 
that the ringfed trolley rectifiers added 
receiver noise and that additional rec- 
tifier line filtering helped but did 
not eliminate the local problem when 
the trolley was near the rectifier 
stations. 

External audio noise and replacing 
the internal battery approximately every 
90 days were the most annoying problems 
associated with the loudspeaking paging 
phones. 

Although the two systems are elec- 
trically tied together, the loudspeaking 
paging phone was primarily used to reach 
the working section and the carrier phone 
was used for right of way, placing loads 
and empties, personnel location, and 
requesting supplies. When the trolley- 
mounted carrier phone was used for self- 
traffic control, the operator would twice 
give his location and destination and 
then proceed to his final destination. 
Although this method of traffic control 
worked for this mine, an improvement 
could be seen using a dispatcher. 

Tape recordings for a first 8-hour 
work shift survey show that the most fre- 
quent call made was concerned with right 
of way. Also, it was noted that out of 
the total 296 calls on the trolley and 
pager system, only 8 were on the pager. 
By listening to tape recordings of both 
the pager and the carrier phone simulta- 
neously, it was discovered that 78 calls 
out of the 296 were not heard on the out- 
side trolley carrier phone. It was also 
found that the trolley-to-pager hookup 
failed on 12 occasions. The actual fail- 
ure of the coupler to function on some 
signals and the propagation dead zones 
were major problems associated with this 
system. 



179 



During the fire previously men- 
tioned, the carrier phones were the only 
communication that worked through the 
evacuation. The telephone wire was 
fused, and communication was not possible 
using the dial system. However, the car- 
rier phones could and did operate, using 
their internal batteries, through the 
fire. Although the loudspeaking phones 
were not in widespread service at the 
time of the fire, their line would have 
also been fused, making them inoperative 
if and when needed. 

Communications Requirements — User 's 
Viewpoint 

Mine personnel indicated that the 
most urgent communication requirement was 
the elimination of dead zones in their 
trolley carrier phone system. 

Communication between the shuttle 
cars and from the shuttle cars to the 
continuous miner was also thought to be 
useful. However, there was apparently no 
great need or requirement for this type 
of comminication. 

Portable two-way wireless communica- 
tion for the maintenance foreman, fire 
boss, miners on the weekend inspection, 
and working section foreman was noted as 
a possible requirement. If portable two- 
way wireless equipment costs were high, 
the maintenance foreman, roving super- 
visors i and key personnel could use a 
one-way pager. However, mine personnel 
did not consider equipping a working sec- 
tion with a one-way pager since a working 
section foreman mostly communicates 
out from his location and is seldom 
called from other sections or surface 
locations. 

A requirement existed for battery- 
operated portable emergency communica- 
tions that could be moved with the miners 
as the working section moved. This 
requirement became evident during the 
fire, when it would have been useful dur- 
ing the emergency and recovery efforts. 
Also noted was the fact that if the num- 
ber of working sections increased. 



the mine may economically justify a 
dispatcher. 

Communication Requirements — Based 
on Survey Analysis 

This mine is unique in that it has a 
dial telephone system, a pager system, 
and a carrier phone system. The mine, as 
configured, has no need for additional 
channels, private channels, or the capa- 
bility to interconnect to the public 
phone since the dial telephone has all 
these capabilities. 

From the traffic density seen on the 
pager phone and trolley carrier phone 
system, only two additional channels can 
be justified to get the probability of 
blocking to the 1% level. However, only 
3% of this communications is from the 
pager phone system, and the vehicle com- 
munication system must be single-channel 
operation for safety reasons. Therefore, 
there exists no justification for addi- 
tional channels for the pager phone. 
Using this analysis and the needs gener- 
ated by the mine personnel, the following 
list was developed to represent commini- 
cation requirements for this mine. 



a. Reliable two-way 
communication. 



vehicle 



b. A dispatcher with comminication 
center if the mine increases appreciably 
in size. 

c. Portable two-way wireless com- 
munication for working section foreman, 
maintenance foreman, and key personnel. 

d. Portable battery-operated com- 
munication equipment for mine-to-surface 
emergency two-way communication. 

A. 5 Mine D 

Mine Description 

Mine D was opened in 1892. Even 
though the mine is old, they are still 
developing in some areas. At present 
they have eight sections on development 



180 



and five on retreat. It is estimated 
that there are 15 years of mining left. 

The mine is entered through one of 
two drift mouths. A vertical shaft is 
available but is seldom used. The mine 
produces approximately 4,500 tons of coal 
a day by the conventional, continuous, 
and longwall mining methods in the fol- 
lowing percentages: 

First shift: 

Conventional 18 

Continuous 35 

Longwall 10 

Other shifts 37_ 

Total 100 

The coal is removed from the mine by 
track to the cleaning plant about 2 miles 
from the mine. The mine has an annual 
production of about 1,300,000 tons. 



is taken to the same cleaning plant as is 
the coal mined underground. This 
requires that dispatcher 1 dispatch 
right-of-way outside as well as 
underground. 

The mine employs a total of 675 men 
and works three 8-hour shifts each day, 
with the major production done on the 
first shift. The start and end of each 
shift and the total men working follow: 



First shift: 



6:45 a.m. to 3 p.m. 
308 men 



Second shift: 3:30 p.m. to 

11:45 p.m. — 216 men 



Third shift: 



12: 15 a.m. to 
8:30 a.m. — 151 men 



Mine Equipment and Power 



The mine is 7 miles by 5 miles and 
has overburden from 300 feet to 800 feet. 
All haulage tunnels are at least 6 feet 
high by 18 feet. There is no average 
size for a working section; some are as 
much as 3,000 feet in length. This 
requires that coal be removed from the 
working face to the track by belt. The 
belt is 36 inches in width, and the mine 
has approximately 25,000 feet of belt. 
If the distance is short (less than 
500 feet), it is possible to dump coal 
from the shuttle car directly into the 
coal car. 

Eight 37-ton locomotives are used to 
remove the coal. The mine has 30 miles 
of track underground at present. Eight 
boreholes are used to provide access for 
13,800-volt-ac, three-phase, cables to 
the mine. Roof control is obtained by 
the use of roof bolts ranging in length 
from 42 inches to 8 or 9 feet. 

Owing to the amount of track and 
layout of the mine, it is necessary to 
have two dispatchers. This mine is also 
engaged in strip mining at various loca- 
tions directly above the mine. This coal 



Power is provided by the power com- 
pany at 138,000 volts ac. This is then 
stepped down to 13,800 volts ac and dis- 
tributed to eight boreholes, where it is 
taken underground. At some point under- 
ground it is converted to 250 volts dc, 
550 volts dc, or 440 volts ac three- 
phase, depending on the equipment being 
used on the section and location in the 
mine. One of the reasons for this is 
that from the outside to dispatcher 2, 
the mine uses 550 volts dc on the 
trolley. Then branching out from dis- 
patcher 2, the mine uses 250 volts dc on 
the trolley. The 440 volts ac is used in 
both areas. 

The power provided to sections var- 
ies according to the type of equipment 
used. There are cases where, on the same 
section, 250 volts dc or 550 volts dc 
must be provided for the shuttle cars, 
and 440 volts ac provided for the miner. 
Both longwalls require 440 volts ac, as 
do some of the newer continuous miners. 
There are also battery-powered scoops on 
some sections. They are used for clean- 
ing the section and hauling supplies. 



181 



The mine at present uses the follow- 
ing equipment: 

Continuous miners — 8 

Loading machines — 5 

Longwalls (1 plow, 1 shear) — 2 

Cutting machines — 3 

Shuttle cars — 29 

Present Mine Communications 

At present, the mine communication 
system consists of a magnetophone system, 
carrier phone system, and loudspeaking 
phones. 

The loudspeaking phones are used 
only on the longwalls. Carrier phones 
are placed on most of the track vehicles. 
At the cleaning plant a 60-watt amplifier 
is used as a public address system call- 
ing 14 stations, each of which has a 
microphone and speaker. 

Each dispatcher is responsible for 
carrier phone and magnetophone control in 
his area of the mine. The two dispatch- 
ers must consult with each other when 
routing traffic toward each other. Typi- 
cally, the telephone network having dis- 
patcher 2 as its control point has 
heavier traffic of a more varied nature 
than that of dispatcher 1. 

During the busiest period of the 
shift, the fourth hour, the busier trunk 
was used 70% of the time. This three- 
channel traffic intensity implies that 
there is a 30% chance of getting a busy 
signal on any given call. This is con- 
siderably worse than the one chance in a 
thousand of commercial telephone stan- 
dards. A six-channel network would be 
required to bring the system to commer- 
cial standards. 

There have been no major emergencies 
at the mine to test the existing system. 
It is possible that a roof fall could 
break the phone line and cut off communi- 
cations to the outside for some areas of 



the mine. In the case of an accident the 
section notifies the dispatcher, who 
in turn calls mine management on the 
outside. 

Telephone System 

The mine has 77 magneto telephones, 
60 of which are underground. These 
phones are approximately 30 years old. 
They use simple twisted-pair. No. 14 wire 
for the phone circuit. 

The phones are usually mounted on 
wood that is connected, in some manner, 
to the roof. They are placed at loca- 
tions along the main haulage. Phones on 
the section are located at the head and 
tailpiece of the belt. These phones are 
not permissible. 

The dispatchers are the heart of the 
phone system. Dispatcher 1 is respon- 
sible for the outside phones and for 
underground phones 1 to 20. Dispatcher 2 
has phones 20 to 60. If a person wished 
to call outside from say phone 57, he 
would have to ring dispatcher 2. Dis- 
patcher 2 would then call dispatcher 1, 
who would ring outside, then connect the 
lines. 

Since the phones are a ringer type, 
each station must have a certain ring. 
It should be kept in mind that the cir- 
cuits for the two dispatchers are sepa- 
rated. Therefore, each dispatcher could 
use the same ring combinations. 

Recordings and corresponding analy- 
sis of the traffic on the phone system 
shows that there are periods when the 
system is used 70% of the time and that 
the system is overloaded at times. 

Trolley Carrier Phone System 

The carrier phones are mounted on 
most of the track vehicles. The fact 
that the mine uses 250 volts dc and 
550 volts dc on the trolley requires the 
use of two different carrier phones. 
They have twelve 250-volt, 163-kHz 
trolley phones and twenty-eight 550- 
volt, 100-kHz phones. Both tube and 



182 



transistorized versions are used. The 
tube type is 20 years old, and the tran- 
sistorized type is 12 years old. The 
transistorized units are equipped with a 
12-volt battery, so that they will still 
operate should the power in the mine go 
off. 

Pager Phones 

Pager phones are used on the long- 
walls and on the outside of the mine. 
The pager phones are mounted on J-hooks 
from the roof support jacks. Wires are 
hung from the roof supports for the 
phones. Rocks falling between the jacks 
have caused the line to break, causing a 
potential safety hazard due to inter- 
rupted communication. The reason for 
this is there is no way of hearing a ring 
on the pager system. There are 10 pagers 
on the plow and 5 on the shear. The 
phones are 10 years old. The mine per- 
sonnel felt that the phones were mis- 
treated by man and environment, and that 
was the reason for failures. 

Fan Monitors 

The size of the mines requires that 
the fans be located at great distances 
from the maintenance shop. The fans are 
monitored by sending a signal over the 
high-voltage lines, which is monitored at 
the outside shop. The five frequencies 
used (one for each fan) are 39, 116, 47, 
61, and 33 kHz. 

Communication Requirements — User's 
Viewpoint 

The phone system performed very 
well considering its age. However, the 
changes in humidity caused some problems. 
There was also a problem with having to 
walk long distances. The phones are not 
permissible; this limits how close to the 
working face they can be placed and often 
requires that an individual walk as much 
as 500 feet to reach one. 



individuals on the section. The mine 
personnel were of the opinion that com- 
munications to those two men would be 
desirable. 

The maintenance foreman and master 
mechanic felt that portable two-way com- 
munications would decrease the time 
needed to locate them. Portable communi- 
cations are also desired for the super- 
visory personnel (superintendent, mine 
foremen, and maintenance foremen). At 
the same time a private line was 
requested for the phone system for super- 
visory personnel. 

The safety department personnel sug- 
gested that remote monitoring of the mine 
conditions would help increase safety for 
the entire mine. It was suggested that a 
private channel directly to the outside 
for emergency use would decrease the time 
required to get help from the outside. 
This private channel would also insure 
that the occurrence of an accident would 
not be heard by men on other sections. 

There should be a secure channel 
open at all times, from any phone under- 
ground, to some central communications 
center aboveground. It is not necessary 
that this line be connected to the com- 
mercial phone system. Since the . mine 
management are the first to be notified 
in case of an emergency, they in turn can 
call whomever is needed. In a mine this 
size the time saved by placing the call 
from underground to the commercial system 
for assistance, then notifying manage- 
ment, would be of little help. 

Communications Requirements — Based 
en Survey Analysis 

This mine has some communication 
problems that are related only to the 
extreme age of the equipment used. How- 
ever, problems due to the large size of 
the mine may be typical of other large 
mines. 



Mine personnel felt that wireless 
communications of some type would be of 
help on a section. The foreman and the 
mechanic are the two most sought after 



Signal-to-noise ratio (SNR) causes 
problems when talking great distances (5 
to 10 miles). A new system must start 
by improving SNR on long-distance 



183 



converations , which may be typical of 
many large mines. 

An analysis of the telephone traffic 
density indicates that three more chan- 
nels (total six channels) would make the 
system comparable to an estimated mine 
standard of 1 in a 100 chances of getting 
a busy signal. 



is the "stall machine" used at the tail 
end of the plow longwall to give better 
roof control. This machine is a limited 
travel shear that leaves a cleaner end on 
the longwall than the plow. About one- 
third of the mining is by longwall, one- 
third is conventional, and the last third 
is continuous. The number of working 
sections for each type of mining follows: 



A dial system is recommended for 
this mine. A multipair or multiplex 
system would help to lessen the load of 
the dispatcher and could also provide the 
capability for conference calls. These 
systems also provide the channel privacy 
requested by mine management. For safety 
reasons, the trolley carrier phone system 
should remain one channel. 

Using the above analysis and the 
suggestions of mine management, the fol- 
lowing list of improvements was derived: 

a. Reliable two-way vehicle system. 

b. A total of at least six channels 
to meet minimum standards. 

c. At least one secure channel. 

d. Portable two-way wireless com- 
munications for certain key personnel. 

e. Battery-operated communications 
equipment that will work during an 
emergency. 

f . A communications center located 
at dispatcher. 

A. 6 Mine E 

Mine Description 

Mine E has a drift entry in a 5.5- 
foot soft coal seam. The working sec- 
tions are presently 3.5 miles in from the 
entrance, with the possibility of eventu- 
ally working at twice this distance. 
Mining at the present rate gives the mine 
a life of from 30 to 40 years. The mine 
operates two longwalls about 500 feet 
wide and will travel a range of 
3,500 feet. New to mining in this area 



1st 


2d 


shift 


shift 


2 


2 


2 


2 


1 


1 



Conventional. 
Continuous. . , 
Longwall 



Only a small amount of mining is 
done during the maintenance or third 
shift. The mine is small enough that 
there is no underground maintenance shop. 
Hence, repairs that cannot be made at 
the site of the failure must be made 
outside. 

Coal is moved from the face by 
shuttle car except on the longwalls, 
where it goes directly to belt. Local 
belt haulage is used between the shuttle 
cars and tracked cars on the main line. 
The longest belt run is 4,500 feet. 

During the first shift there are no 
idle sections so there is no need for 
maintenance crews when each section crew 
has its own mechanic. Extra mechanics 
work along the main line during this 
shift and help section mechanics when 
needed. 

During the third shift, when few 
sections are working, there are three 
maintenance crews whose specific job 
is to work on equipment at the idle 
sections. 

Mine Equipment and Power 

Power is fed to the mainline trolley 
wire at 250 volts dc by four rectifiers. 
There are deadblocks between the four 
sections of trolley wire so that each 
rectifier supplies power to only one 
short length of wire. All face-mining 
equipment is ac operated so there is no 



184 



need to have nip points from the trolley 
line. 

Rather than utilizing power bore- 
holes, 7,200-volt ac power is brought in 
along the mainline, up to transformers at 
the working sections. There the voltage 
is stepped down to 440 volts. Thus as 
the sections advance, the transformers 
must be moved to follow. 

The only power outages have been due 
to storms or hunters shooting transform- 
ers on the power company's distribution 
system. Outside there are two high lines 
feeding the mine's single substation. 

Should there be a power interruption 
within the mine, the substation attendant 
will check by telephone with the sections 
before reenergizing the distribution 
system. 

Present Mne Communications 



Telephone System 

Pager phones are used in a network 
of 11 phones along the track throughout 
the mine, plus a phone in the dis- 
patcher's outside office and one at the 
communication repair station in the shop. 
Tape recordings made during an 8-hour 
shift indicate that there were 160 dis- 
cussions concerning the location and 
movement of empty and loaded coal cars. 
For the next most common topic, there 
were nearly 80 discussions concerned with 
the production of mined coal. Collec- 
tively the other subjects (reporting, 
personnel location, maintenance, etc.) 
add up to about 80, so no one of them is 
a significant user of channel capacity. 
Analysis showed that early in the shift, 
and just before the end, the phones are 
used as much as 50% of the time. This 
places the probability of a potential 
caller finding a busy line at one chance 
in two during these periods. 

The loudspeaker telephones have an 
average age of about 4 years. Rock dust 
does seep in through their cases, but the 
users and maintenance men report there 
are few failures and these fall in no 



consistent pattern. The people inter- 
viewed could give no suggestions on how 
the phone system — the one following the 
track — could be improved. There are sev- 
eral reasons that could be contributing 
to their satisfaction: 

a. The phones are relatively new. 

b. The phone network is not large. 

c. The time and talent spent on 
maintaining the system are great. 

d. The equipment supplier gave them 
much help in setting up their systems. 

e. The characteristics of the phone 
lines are good. 

The telephone network is such that 
anyone on a working section is never more 
than 350 feet from a phone. They feel 
this is adequate and that having a phone 
any closer would not really be of more 
value. Other phone locations are the 
boom and tailpiece of every belt, plus 
four in the escapeway. No allowance has 
been made for emergency usage in the 
sense that, should a telephone line be 
broken, there is no loopback to carry the 
signal. 

Carrier Phone System 

The carrier phone is a 72-kHz system 
that uses the trolley power line to carry 
the signals. Even though this is a 
fairly small mine, they did experience 
dead zones of unacceptably low signal 
strength in certain areas. 

The dispatcher has an outside dial 
phone as well as a speaker phone, so he 
serves as a message relay center and 
information center as well as a dis- 
patcher. The communications maintenance 
area of the shop also has a trolley car- 
rier phone to aid the maintenance people 
in servicing the trolley carrier phone 
system. The carrier phones do not have 
storage battery backup. If there is a 
failure of trolley power, carrier phone 
communications are lost. 



185 



The mine has had a dispatcher for 
only the last 2 years. Before that, 
motor operators controlled the track for 
themselves. At that time the mine tried 
tying the telephone system in with the 
trolley phone system (this type applica- 
tion cannot be used with an intrinsically 
safe telephone system) but found it only 
added confusion to have those not near 
the main line hear all the discussions of 
the motormen. 

To get the carrier signal around 
the deadblocks, 2-yF capacitors are used. 

To keep rectifier "hash" out of the 
72-kHz system, L-filters are used at each 
rectifier (paragraph 5.3.1a). The filter 
consists of a SO-pF capacitor across a 
rectifier's output, with a 10-turn, 
heavy -cable coil, the coil having an 
approximate diameter of 2 feet. The 
manufacturer tuned the filter to reject 
72-kHz interference. 



CAUTION 

Installation of equipment in a 
mine should be done only by people 
thoroughly qualified to do such work. 
Installations should follow proce- 
dures recommended by the equipment 
manufacturer and should comply with 
good safety practices. All installa- 
tions should also comply with appli- 
cable codes and regulations. 



Longwall Communication System 

The longwall miner has its own com- 
munications system consisting of seven 
loudspeaking telephones, one at each end 
and the other five equally spaced along 
the 500-foot longwall. These loudspeak- 
ing telephones have no handset and thus 
operate in the pager mode only, with the 
loudspeaker serving as a microphone when 
the push-to-talk button is pressed. The 
telephone lines lie in the troughs that 
carry the hydraulic lines. At one time 
the wires were hung under the top plate 
of the jacks, but slate falling between 
the jacks kept breaking the wires. 



Signal lights are positioned along 
the longwall miner so the operators can 
coordinate their efforts if the phone 
system fails. The quality of speech 
reproduction for the phones was good, and 
the loudspeaker volume was adequate in 
spite of the high ambient noise of the 
miner. 

The only complaint the personnel had 
was that the push-to-talk button failed 
often. This button is mounted on the 
recessed front face of the unit. The 
phones at the ends of the longwall are 
mounted horizontally so the recessed 
panel acts as a catch basin for the 
watered-down coal dust. Evidently the 
directional properties of the speaker are 
such that this mounting is necessary. 

Communications Requirements — 
User's View 

Except for correction of the minor 
problems already presented, the mine per- 
sonnel had little to suggest about new 
communications systems that would make 
their work safer and more effective. 
This may be due to their present system 
being new and seemingly adequate for this 
size mine, or due to their not having 
time to visualize how a higher capability 
system might profitably be utilized. 

The one desire expressed at this 
mine was for a secure channel for seeking 
aid for an accident victim. As in other 
mines, when an accident is being 
reported, everyone who knows the phone is 
being used for this purpose will listen 
if he can. This lowers the productivity 
of the eavesdropper; takes his mind off 
his work, making him more accident prone; 
and worse, loads the telephone system so 
that the dispatcher can no longer clearly 
understand the report. It is not essen- 
tial that the conversation cannot be 
listened to, just that personnel not 
become aware that someone in a panic 
is calling the dispatcher. Personnel 
seldom listen in on run-of-the-mill 
conversations . 



186 



Communications Requirements — Based 
on Survey Analysis 

The exceptional high quality and the 
unusual amount of care given to the tele- 
phone and carrier phone systems leave 
little to suggest as to improving these 
communications means in mines similar to 
this one. 

This mine, like some others visited, 
has a need for a secure channel as an aid 
in effectively handling injury problems, 
and it would be desirable to have a 
secure management channel. 

Better communications capability 
would increase productivity in the long- 
wall mining sections. Fast, effective 
hands-free communication is needed by 
operating personnel during both operation 
and repair of the miner. Because of its 
high production rate — and thus the high 
cost of downtime — and because of the 
almost impossible working conditions, it 
seems essential that all longwall workers 
have their own wireless communications 
network with each having small, light- 
weight equipment, including speakers and 
bone-conducting microphones mounted in 
helmets. The communication center oper- 
ator should also be able to monitor this 
network. 

A. 7 Mine F 

Mine Description 

Mine F has been operational since 
1963 with production originally estimated 
for 25 years. Coal is being mined 
from the Mammoth Seam in the Cherokee 
Group. Seam thickness is approximately 
60 inches. 

This mine is the only nonunion mine 
surveyed. As a result, some of the oper- 
ations are notably different from those 
seen at the other mines examined. 

The mine employs conventional mining 
techniques and employs tracked haulage to 
remove the coal. Personnel entry and 
coal removal are through a single shaft. 



Coal production is approximately 
250,000 tons per year. There Is one rain- 
ing section, operating one shift. Coal 
is mined via the room and pillar method 
with activity rotating through six active 
rooms . 

The mine will ultimately be approxi- 
mately 1-1/4 miles square. Mining activ- 
ity is currently occurring about 3/4 mile 
from the shaft. 

The overburden at the shaft is 
157 feet, increasing gradually to the 
working face. Tunnels are typically 
12 feet wide and range from 4 to 6 feet 
high. Four- and 6-foot roof bolts are 
Installed on 5-foot centers. 

There are no boreholes into the 
mine. The fresh air entrance serves as 
the emergency exit and is located about 
500 feet from the main shaft. The main 
shaft serves as the air exhaust. 

Mine Management 

Since there is only one raining 
section, the mine operates with very 
few management personnel, as follows: 
General manager, chief engineer, super- 
intendent, and foreman. 

Management personnel do quite a bit 
of filling in as necessary; however, the 
chief engineer normally tends to topside 
operations while the superintendent 
stands by at the bottom of the shaft. 
The foreman remains at the face. The 
mine has 25 men underground during the 
shift. 

There are five mining operations 
rotating continuously through the six 
active rooms at the face. A cycle starts 
with the cutter undercutting the coal 
face. This is followed by the coal 
driller drilling holes for the charges. 
After the driller moves on, the charges 
are set and fired by the shot flrer. 
After a delay for the air to clear, the 
loader is moved in to begin loading 
shuttle cars, which transfer coal to the 
haulage cars. When a room has been 



187 



cleaned of the loose coal, the roof bolt- 
ers move in to extend the supported sec- 
tion of the roof. 



Loaded haulage trains are pulled to 
the shaft where the cars are dumped into 
a skip, one car to a skipload. The skip 
is lifted up the shaft and dumped into 
the crusher. Crushed coal is conveyed 
into semitrailer trucks that are used 
exclusively to haul the mine's output. 

The maintenance philosophy of this 
mine results in a large amount of nonpro- 
ductive machine time. There is a com- 
plete operating spare for each type of 
machine in the mine. As a result of this 
philosophy, however, there is virtually 
no downtime for equipment maintenance. A 
minor failure is repaired on the spot; in 
case of a major failure, the spare 
machine is placed in service while the 
broken one is fixed. 

There is no fixed shop location. 
The maintenance personnel travel with the 
mining crew. The presence of spare 
machinery permits repairs and maintenance 
to be performed thoroughly without slow- 
ing production. 

Supplies are delivered via the haul- 
age cars. Just before the end of each 
working day the foreman calls his list of 
supplies to the hoisting engineer. These 
are placed at the top of the shaft and 
delivered to the face either at the end 
of the day or the beginning of the next 
one. Repair parts are delivered during 
the day via a return trip of the haulage 
cars. 

The mine has a single man-trip car. 
This is sufficient to carry the entire 
crew, so only one man-trip is made, morn- 
ing and evening. Administration of the 
mine operation is quite informal. The 
general manager oversees all operations 
and assists the topside personnel as 
necessary. All management personnel 
assist when and where needed. 



Ventilation is via a single fan, 
blowing into the escape shaft and 
exhausting through the main shaft. 
Within the mine, water sprays are used to 
keep dust down. There has never been any 
problem with excessive water, so the only 
water-handling gear is that used to con- 
trol dust. 

Mine Equipment and Power 

The following pieces of mining 
equipment are in use at the mine: 

Shuttle cars — 3 Roof bolter — 1 
Loader — 1 Coal drill — 1 
Cutter — 1 Locomotives — 3 

In addition to the equipment in use, 
there is one operating spare of each type 
of machine. In case of major breakdown 
the spare is placed in operation while 
the broken unit is repaired. 

Primary power comes into the mine 
through the main shaft. A 2,300-volt, 
three-phase line is run to the two 
transformer-rectifier sets used. One 
transformer feeds the trolley for the 
haulage system; the other powers all 
machinery at the face. All machines in 
the mine run off 280 to 300 volts dc. 

Present Mine Communications 



The mine currently has a combination 
of three independent voice communication 
systems. The loudspeaking phone system 
uses two units, one located at the hoist- 
ing engineer's position, and the other at 
the working face. A two-station intercom 
connects the top and bottom of the shaft. 
Another intercom connects the hoisting 
engineer and the mine office. Two spare 
loudspeaking pagers serve as backup and 
permit a third station to be patched in 
if work is being done a long way from the 
face. The hoisting engineer serves as 
"communications central," tying the three 
systems together. In addition to the 
internal communication systems, an exten- 
sion of the outside telephone line is 



188 



located at the chief engineer's desk. 
The pager at the face is kept mounted 
near the power sled, so the two are moved 
together. Nothing else is moved. 

All equipment has been holding up 
well. Perhaps twice a year, one of the 
pagers will quit operating. Whenever 
this happens, the bad unit is removed and 
sent to a commercial repair station. 

In normal system use, all calls are 
made from a remote point to the hoisting 
engineer "communication central." As 
long as calls are being made in this man- 
ner, the system functions well. A pos- 
sible exception might occur in an emer- 
gency situation at the face. The pager 
at this point is 50 to 100 feet from the 
nearest working room, and on the other 
side of air-diverter flaps. It is con- 
ceivable that an accident could occur in 
which the phone would not be accessible. 
The other possibility involving an acci- 
dent situation would involve the phone 
cable. There is a single run with no 
backup or loopback path. This cable is, 
however, protected in being mounted on 
vertical timbers and is of armored 
construction. 

When calls are made from the "com- 
munication central" position to other 
parts of the mine, the system does not 
work so well. A complaint was made that 
if the superintendent leaves the bottom 
of the hoist it may take a half hour to 
get a message to him. It appears very 
unlikely that a call to the pager at the 
face would find anyone near enough to 
hear it. The fastest route to the face 
appears to involve relaying a message to 
a motorman at the hoist and having him 
deliver it to the face when he returns. 

In this mine, communications ef- 
ficiency would be improved by replac- 
ing the three independent two-station 
phone systems with a single multistation. 



multichannel system. The system should 
have a minimum of seven stations. 

Their locations would be as follows: 

Mine office 

Hoisting engineer's position 

Shaft bottom 

Working face 

Bottom of emergency shaft 

Midway between bottom and face along 
inbound haulageway 

Midway between bottom and face along 
outbound haulageway. 

Other stations that might be con- 
sidered include — 

Topside storage or shop area 

Chief engineer's desk 

Crusher 

Most of the added stations would be 
concerned more with safety than with 
production. As things stand, it is 
possible to be blocked from a phone sta- 
tion or to be a long walk from one. In 
addition to the fixed stations, the man- 
agement personnel should have radio com- 
municators. This would eliminate the 
existing situation in which a critical 
man can be out of touch when others need 
a decision or information. 

An expanded system needs no more 
than two general channels plus a private 
channel. Radio communicators operate 
best if they can access all three chan- 
nels, but could operate with access only 
to one of the general channels. 



189 



The cable into the mine should be a 
continuous loop of armored wire for maxi- 
mum reliability and protection. By using 
a multiplex system, all channels plus 
monitors could be included on a single 
cable. 1 

A. 8 Mine G 

Mines that do not employ rail haul- 
age systems powered from a trolley wire 
face unique problems in establishing sat- 
isfactory communications between haulage- 
way vehicles. Because common trolley 
carrier phones cannot be utilized, some 
other form of radio system must be used 
to establish the required voice link 

^Approved and nonapproved systems may 
not share the same cable; check with MSHA 
for details. 



between motormen and/or motormen and a 
central dispatcher. This description 
illustrates how one mine in this category 
solved its haulageway communication 
requirements using a unique system of UHF 
and VHF repeaters combined with a "leaky 
coaxial" transmission line. 

This mine was a magnetite ore block 
caving operation. Surface access to the 
Mine (fig. A-2) was by two vertical 
shafts to the No. 6 production level, 
2,500 feet, with mining occurring at a 
depth of 2,500 to 2,800 feet. Diesel- 
powered, rubber-tired loading equipment 
was used to transport ore to the crush- 
ers. A conveyor belt ran between the 
crusher rooms and the ore skip storage 
bins where the ore was automatically 
loaded into 20-ton skip cars and hoisted 
to the surface. 



No. 2 crusher 

Fan hole drill operator 




No. I crusher 



Shop 

°^*"'' 4 shaf? 



B shaft 



; 



FIGURE A-2. - Underground map of mine, 6th level 



190 



Personnel underground included rov- 
ing miners in production, haulage, and 
shop areas, fan-hole drill operators 
working alone, and maintenance vehicle 
and ambulance operators. To satisfy the 
objective to communicate between these 
personnel and the surface, a guided wire- 
less communication system utilizing 
equipment available from Motorola, Inc. , 
and Andrew Corp. was selected. Portable 
HT-220 radios and industrial dispatcher 
mobile transceivers were chosen for 
personnel and vehicles, respectively. 
Andrew Radiax cable, a special type of 
cable that allows for leakage of signals 
out of and into itself, was installed 
throughout the major areas of the mine. 
Because the total cable length exceeded 
2 miles, it was necessary to install two 
repeaters. Although the system did not 
require a dispatcher or an operator, a 
communications center was established at 
an underground crusher room. Personnel 
could be selectively paged from the con- 
sole, and an evacuation alarm could be 
activated from either the console or an 
alternate monitor station at the shaft 
bottom. The monitor station was wired to 
the surface where a remote control unit 
provided surface access to the system. 
During a power failure, the system would 
operate for 24 hours on backup battery 



power. A telltale beep in the system 
signaled that the system had reverted to 
emergency power. The communication sys- 
tem utilized off-the-shelf, readily 
available communications equipment and 
installation hardware. In addition, the 
system was compatible with the installa- 
tion and maintenance capabilities of the 
mine personnel. 

Figure A-3 shows the extent of the 
radiating cable installation. There was 
11,000 feet of RX5-1R 7/8-inch Andrew 
Radiax in the system. The cable attenua- 
tion was 1.2 dB/100 ft; thus, two re- 
peaters were required to compensate for 
signal loss as well as provide adequate 
power levels for future system expansion. 

The cable specifications state that 
it must be supported every 5 feet. To 
avoid installing 2,000 anchors in the 
rock, a 3/16-inch steel messenger wire 
was attached at 20-foot intervals to 
roof -bolt-supported T-bars. The cable 
was then strapped to the messenger wire 
with standard cable ties. 

Some areas were so far away from the 
installed cable that radio transmissions 
could not be established. This was over- 
come by inserting a stub cable with one 



3 — Disporcher console 
^epeorer A 



LEGEND 
i&^ Power divider 
>- Antenna 
O Repeater 
A Fixed station 




Scale, ft 



Alrernare 

control console 



'I 



Shaft A 



FIGURE A-S. - Leaky feeder cable layout. 



191 



end connected through a power divider to 
the main cable; the other end terminated 
in an antenna located within several hun- 
dred feet of the desired working area. 

The repeaters, composed of a unique 
combination of UHF and VHF units, were 
bolted together and mounted on pallets 
for ease of transport within the mine. 
The UHF and VHF units were interconnected 
by a squelch and audio interface. Vehi- 
cular and personnel communications took 
place over the leaky coaxial cable on the 
UHF repeater frequencies, while the con- 
trol between repeaters, located some 
2,000 feet apart, used VHF over the same 
cable. A 5-MHz UHF transmit and receive 
frequency separation allowed connection 
to the common Radiax through a duplexer. 

The repeaters were prewired, and the 
system was assembled on the surface where 
it underwent several months of burn-in. 
This procedure eliminated the frustra- 
tions of troubleshooting and testing the 
system underground. 

The fan-hole drill operator, 
equipped with a portable radio attached 
either to his belt or to a chest pack as 
he preferred, was also equipped with an 
accessory noise-reducing earrauff and mi- 
crophone. The receiver audio was routed 
to small loudspeakers inside his ear pro- 
tectors, while the microphone and push- 
to-talk switch were installed in a simi- 
lar "earmuff which the fan hole drill 
operator placed over his mouth when he 
wished to make a radio transmission. All 
portable radios in the mine were equipped 
with the provision to use an external 
speaker microphone accessory so that the 
radio need not be detached from the oper- 
ator's belt and raised to his ear or 
mouth. In noisy locations the use of a 
noise-reducing speaker-microphone is a 
necessity. 

Two types of vehicular radios were 
used. The Industrial Dispatcher had all 
controls, the microphone plug, and 
speaker located on the transceiver pack- 
age; this necessitated locating the 
transceiver within the vehicle operator's 
reach, which is nearly impossible on some 



mining vehicles. A better radio for this 
application was the motorcycle version of 
the Industrial Dispatcher. All controls 
and the microphone plug were located on 
the small, rugged loudspeaker enclosure. 
The loudspeaker can be mounted in a 
convenient location, and the larger 
transceiver unit can be mounted in a more 
protected location. The antennas are 
either 1/4-wave whips or Sinclair low- 
profile blade antennas. The radome 
version of the blade antenna appears 
to be the most suitable for mining 
applications. 

A dispatcher control console at the 
No. 2 crusher could be either manned or 
unmanned; no operator was necessary for 
system operation. Paging could be initi- 
ated from an encoder at the console to 
send private messages to pager-equipped 
radios. Equipping the fan-hole drill 
operator with a pager-encoded radio pre- 
vents the nuisance of his listening to 
general system traffic. He would only 
hear messages directed specifically to 
him. The console also had the provision 
for sending a warning signal to all 
radios in the mine. This wailing siren- 
like signal could be used for mine evacu- 
ation in an emergency. 

The alternate control station pro- 
vided an additional access point to the 
communications network. This station 
monitored the activity of the fan-hole 
drill operator. Also, this station was 
connected by wire line to an intercom 
unit in the surface guardhouse. The 
guard could access the system through 
remote control. This feature was espe- 
cially desirable during maintenance peri- 
ods when the underground stations are 
unmanned or during a mine emergency, 
to coordinate evacuation and rescue 
operations. 

The mine had outfitted an under- 
ground radio shop with the necessary 
service equipment to maintain the commu- 
nications system. A full-time Federal 
Communication Commission second-class 
licensed radioman was trained in system 
installation, operation, and maintenance. 
Reliability of the UHF-VHF system was 



192 



excellent with negligible downtime. This 
wireless communications system demon- 
strated that the objectives of personnel 
and vehicular underground mine communi- 
cations can be satisfied. Worker and 
management acceptance of the system was 
excellent. 

A. 9 Mine H 

To meet changing communication 
requirements in many of its mines, one 
utility company has installed a new mul- 
tichannel mine dial-page phone system at 
its underground operations. The first of 
its type, this fully permissible comiaini- 
cation system combines the paging capa- 
bility of current mine page phones with 
the advantages of conventional tele- 
phones. Manufactured by GAI-Tronics 
Corp. , the Mine Dial-Page Phone System 
(MDP) has the following features: 

1. Each underground station is on a 
separate circuit ready for instant use 
depending upon the availability of an 
open channel in the central switch. 

2. When connected to a telephone 
switchboard through a 12- to 48-volt in- 
terface circuitry card provided for each 
line, any underground station can call 
another underground station directly, or 
call any standard telephone at a surface 
location. Also, any underground station 
can be called from any aboveground stan- 
dard telephone. 

3. Selective paging capability to 
any single, specific underground station. 

4. A dial-access, all-station pag- 
ing capability to call personnel not at 
their normal location, or to alert all 
underground personnel. 

5. Automatic switching to a push- 
button-operated, all-page-partyline mode 
in the event of a telephone switchboard 
power failure or severance of the cable 
interconnecting the switchboard and the 
interface cabinet, one of the key compon- 
ents of the MDP system. 

6. Plug-in electronic assem- 
blies, wherever possible, to facilitate 



maintenance and adaptation to changes in 
mine operations. 

The MDP system (fig. A-4) consists 
of individual phone stations placed at 
selected sites within the mine, one or 
more interface cabinets located on the 
surface, a telephone switchboard, and the 
necessary raulticonductor interconnecting 
cable. (One pair of conductors is 
required for each private line.) 

Each phone station is contained 
within a bright yellow, molded polyester- 
fiberglass-reinforced housing. This 
material, coupled with the use of stain- 
less steel hardware, gives a corrosion- 
free enclosure. The station includes a 
handset, a telephone dial, a loudspeaker, 
an all-solid-state plug-in amplifier, and 
a self-contained battery of the standard 
12-volt mine page phone type. 

In addition, since some power for 
system operations is supplied from 
the surface, the mine phones are designed 
to have individual power to permit 
emergency communications in the event of 
a power cutoff. This is accomplished 
by a standard 12-volt phone battery, 
while the surface equipment is provided 
with a 12-volt rechargeable battery, re- 
quired only in the event that volt- 
age to the dc power supply should be 
lost. 

Located outside each cable entry 
into the mine is an interface cabinet. 
The circuitry that converts the telephone 
switchboard voltages (ac and dc) to per- 
missible levels is contained in this 
cabinet on a separate plug-in inter- 
face card for each telephone line. 
The cabinet also contains the 12-volt 
rechargeable battery. This battery auto- 
matically powers the system if there is a 
power failure at the switchboard or in 
the connecting cable between the switch- 
board and the cabinet. 

The switchboard itself is an impor- 
tant link in the MDP operation. 

This mine's initial installation 
uses a private automatic branch exchange 
provided and installed by the local 



193 




FIGURE A.4. - Typical mine dial-page phone system. 



194 



telephone company in the main office 
building at the mining operation. 

An incoming call triggers the inter- 
face card circuitry in the cabinet, which 
begins with the activation of a timed 
holding circuit that completes the dc 
loop of the telephone line and halts the 
incoming ringing signal. The timing cir- 
cuit holds the line for approximately 
40 seconds and initiates additional cir- 
cuitry which produces a distinctive 
warble ring tone on the appropriate MDP 
phone. The ring tone is applied for a 
4-second period, and the balance of the 
40 seconds is held for the calling party 
to page a specific person or make an 
announcement. At the end of this period, 
if the called station has not answered, 
the lines are automatically disconnected. 

When the station answers before the 
end of the 40-second hold period, the 
timing circuitry is returned to its 
original standby state and the loud- 
speaker is muted. The party called 
responds by taking the phone handset from 
its holder and squeezing a press bar 
located in the center of the handle. 
Holding of the telephone line is accom- 
plished by a circuit not associated with 
the timing circuit, and the connection is 
held as long as both parties are pressing 
their respective press bars. 

For outgoing calls, the user of the 
MDP phone simply removes the handset from 
the holder and presses the press bar. 
When the familiar dial tone is heard, he 
can dial his call. Release of the 
press bar terminates the connection. A 
delay circuit is provided to maintain 
the line connection during any brief 
(2-second maximum) release of the press 
bar, such as to change hands. 

The aforementioned procedures allow 
one party to call another at a specific 
location. A separate feature is provided 
to page a person when his location is 
unknown. By dialing a special number, a 
separate amplifier and electronic source 
within the interface cabinet activate the 
loudspeaker at each MDP station to pro- 
vide one-way paging communication. Such 



a page call will be heard in the handset 
receiver by all parties engaged in calls 
to, from, and between MDP phones, but it 
will not interrupt these conditions; the 
conversation can continue at the con- 
clusion of the page. The person being 
paged, however, must dial the person 
initiating the page to carry on a regular 
conversation. 

The interface cabinet contains a 
separate fail-safe system to maintain 
conmiunications in the event of an acci- 
dental disconnection of the cable between 
the cabinet and the telephone switch- 
board, or if there is a failure of the 
switchboard's power. A second circuit 
network, controlled by a switchboard mon- 
itor, automatically ties all of the 
interface cards together in the event of 
such failure. In this mode, two-way pag- 
ing and handset conversation can be 
carried out in a manner similar to that 
of presentday mine page phones. A push- 
to-page button is provided for paging in 
this mode, with each phone having its own 
battery to provide power for both normal 
and this alternate-mode operation. 

Ability to dial outside calls — 
including direct-dial long-distance 
calls — and to receive similar calls is 
limited only by the telephone switch- 
board. That is, an MDP phone station can 
be used to dial any telephone or receive 
any incoming call that a conventional 
telephone connected to the same line 
could handle. 

A. 10 Mine I 

Mine Description 

Mine I is a silver mine centrally 
located in the Couer d'Alene mining dis- 
trict in Idaho. The mine was first 
opened in 1884 and presently employs over 
500 persons, 400 of whom work under- 
ground. Main access to the mine is 
through a 200-foot-long adit to shaft A 
at the western edge of the mine. A miner 
proceeds down that shaft to the 3100 and 
3700 levels and then eastward through 
5,000-foot-long drifts to shaft B, which 
is collared at the 3,100-foot level. 



195 



Miners must then go down that shaft to 
the active working levels (fig. A-5) . 

Shaft B is bottomed just below the 
6,000-foot level. Ore is being produced 
on the 4000, 4200, 4400, 4600, 4800, 
4000, 5200, and 5400 levels. Level 
development is in progress on the 
5600 level, and shaft station development 
is in progress on the 5800 level. 

The A and B shafts are each provided 
with electric-powered double-drum hoists 
and electric-powered single-drxim chippy 
hoists. The double-drum hoists on both 
shafts are used primarily for hauling ore 
and waste materials. The chippy hoist on 
shaft A is used for moving men and 
materials to all levels as far down as 
the 4000 level and for hoisting ore from 
the 4000 level to the 3100 level. The 
shaft B chippy hoist is on the 3700 level 







< 
< 

X 




-J 

g 


1900 








L 










il 




3100 


3000 

/ 








/ 






y 










I 
'J 

d 












3400 






gl 










3700 






y 










3700 


y 






















4000 












40O0 




. 












4200 


O 

I 












i 


IT 
O 
















1 4400 


^ 












O 

z 














4600 


4800 












4800 












5000 




1 








5200 










5400 


LEGEND 










I ■ ESCAPE ROUTES 










anno best wanway 






_5600 


























z 


_5800 









FIGURE A-5. - Mine I, mine map. 



and is equipped with a four-deck man cage 
with a total capacity of 48 men. It 
is used for servicing all levels below 
3700. 

Airflow for the mine is dependent 
upon pressures developed by fans located 
underground (series ventilation). All of 
the intake air for ventilation of the 
mine is coursed down shaft A to the 3100 
and 3700 levels. The air is split 
between these two levels and travels lat- 
erally to shaft B. The air is then 
forced down the shaft B to the lower lev- 
els. The return air flows back through 
ventilation raises and exhaust airways to 
the surface. 

The ore deposits occur as long, gen- 
erally narrow veins containing sulfides 
of silver, copper, lead, and antimony in 
a carbonate quartz gangue. The vein dips 
range from 45° to 90° and are generally 
to the south. Strike lengths on the 
major ore shoots range up to a known max- 
imum of 2,200 feet and are normally ex- 
ceeded twofold or threefold vertically 
along the dip of the structure. The 
true vein width varies considerably 
but generally averages between 2 and 
5 feet. 

The steeply dipping fissure veins 
are mined by the horizontal cut and sand- 
fill method by either breasting down or 
back stoping. Stopes are developed a 
maximum of 100 feet along the strike of 
the vein. Level intervals are 200 feet. 
A raise climber is used to drive the 
6- by 6-foot raises between levels. 

All underground transportation is 
accomplished using either the hoists or 
battery-powered locomotives on narrow- 
gage tracks. The mined ore is trans- 
ported to a muck pocket on the associated 
haulage level. This ore, or muck as it 
is called, is then dumped onto the 
shaft B hoist skips and transported to 
the 3100 level. The muck is then trans- 
ported by locomotive to shaft A and 
hoisted 3,100 feet to the headframe stor- 
age bins. 

Surface facilities include an office 
area, warehouse electric shop, machine 
shop, hoist and compressor house, garage. 



196 



carpenter shop, mine and mill changehouse 
for employees, dispensary, and tailing 
ponds. Engineering personnel are also 
located at the mine to provide facility 
planning and better control progress of 
the mining operations. 

Present Mine Communications 



The telephone permissibility re- 
quirements are not applicable to metal 
and nonmetal mines such as this mine. 
A high-dc voltage on the carrier pair, 
although a potential safety problem, is 
of much less severity in a metal or non- 
metal mine. Therefore, an Anaconda S6A 
system was installed to provide telephone 
service underground. 

The Anaconda S6A is a seven-channel, 
frequency division multiplex system. The 
following items are worth noting in 
regard to its suitability for mine 
applications : 

1. The system provides a suitable 
number of channels (seven) on a single 
wire pair. 

2. The mechanical and environmental 
specifications indicate the ability to 
operate under the severe conditions found 
in the mine. 

3. The system allows branches to 
individual conventional dial telephones 
at any point on the system. 

4. Remote units (at the telephones) 
have batteries that are trickle-charged 
over the wire pair. This enables the 
system to be freestanding and not con- 
nected to 110-volt power underground. 

5. Carrier levels require no 
adjustment, as the system has automatic 
gain control circuitry. 

The Anaconda S6A system is designed 
to interface a central office at one end 
and conventional telephones at the other. 
It was designed as a transparent sub- 
stitute for copper pairs connecting the 
telephone company office to subscriber 
telephones. To perform its signaling 



functions, the system receives central 
office signals at one end (such as the 
ringing voltage generated by the central 
office to ring the telephones) and repro- 
duces them at the other end (it remotely 
generates ringing voltages to ring the 
bells as needed). Conversely, the system 
can receive only dial pulses from the 
telephone end, which it passes to the 
central office. When used in this way, 
the system is a transparent substitute 
for copper pairs; that is, users cannot 
tell whether the S6A system or copper 
pairs are being used. 

The central switching function of 
the phone system is handled by a small 
private branch exchange. System require- 
ments were carefully examined before 
choosing a location for this PBX. A 
spare single twisted pair was available 
from the shaft A surface to deep within 
the mine. Any additional wiring in the 
shafts was to be avoided. An air- 
conditioned room was available in the 
shaft B area at the 3,700-foot level that 
met all environmental requirements of the 
PBX. Additionally, this location was 
approximately centered with respect to 
the number of telephones desired in the 
system. The single twisted pair was 
opened at this point, thereby forming two 
independent wire pairs. Carrier termi- 
nals were then installed on each pair, 
and these two independent carrier systems 
were then connected to the PBX circuits. 
This provided up to 14 private channels 
for communication within the mine. One 
channel in each carrier system was desig- 
nated for use in a monitor-control sys- 
tem. Of the remaining 12 channels, 5 in 
each carrier system are used to connect 
phones to the PBX, and the additional 
channel is reserved as a spare. Addi- 
tional phones for critical locations and 
functions in the 3700 level shaft B area 
are directly connected to PBX line cir- 
cuits to provide them with private line 
service. This minimizes the possibility 
of getting a busy signal for these 
phones . 

Each phone has battery backup that 
will allow operation for 24 hours. 



APPENDIX B.— FEDERAL REGULATIONS 



197 



The following sections of the U.S. 
Code of Federal Regulations, Title 30, 
Mineral Resources, Chapter 1 — Mine Safety 
and Health Administration are presented 
to assist planners of communication sys- 
tems in insuring that all requirements 
are being satisfied. 



It should be noted that some States 
have enacted laws that further regu- 
late the use of communications, control, 
and monitoring equipment in underground 
mines. Check State and local regulations 
before proceeding with the installation 
of new or redesigned equipment. 



PART 57— SAFETY AND HEALTH 
STANDARDS— METAL AND NON- 
METALLIC UNDERGROUND MINES 

• 

§ 57.1 Purpose and scope. 

The regulations in this part are pro- 
mulgated pursuant to section 6 of the 
Federal Metal and Nonmetallic Mine 
Safety Act (30 U.S.C. 725) and pre- 
scribe health and safety standards for 
the purpose of the protection of life, 
the promotion of health and safety, 
and the prevention of accidents in un- 
derground metal and nonmetallic 
mines which are subject to that Act. 
Each standard which is preceded by 
the word "Mandatory" is a mandatory 
standard. The violation of a manda- 
tory standard will subject an operator 
to an order or notice under section 8 
of the Act (30 U.S.C. 727). Those regu- 
lations in each subpart appearing 
under the heading "General— Surface 
and Underground" apply both to the 
underground and surface operations of 
underground mines; those appearing 
under the heading "Surface Only" 
apply only to the surface operations of 
underground mines: those appearing 
under the heading "Underground 
Only" apply only to the underground 
operations of underground mines. 



57.11-54 Mandatory. Telephone or other 
voice communication shall be provided be- 
tween the surface and refuge chambers and 
such systems shall be independent of the 
mine power supply. 



i 57.18 Safety programs. 

General— Surface and Underground 



57.18-12 Mandatory. Emergency tele- 
phone numbers shall be posted at appropri- 
ate telephones. 

57.18-13 Mandatory. A suitable coimnu- 
nlcation system shall be provided at the 
mine to obtain assistance in the event of an 
emergency. 



§ 57.19 Man hoisting. 



1 57.11 Travelways and escapeways. 
Travelways 
general— surface and underground 



Hoisting Procedures 

57.19-55 Mandatory. When a manually 
operated hoist is used, a qualified hoistman 
shall remain within hearing of the tele- 
phone or signal device at all times while any 
person is underground. 



198 



Signaling 

57.19-90 Mandatory. There shall be at 
least two effective approved methods of sig- 
naling between each of the shaft stations 
and the hoist room, one of which shall be a 
telephone or speaking tube. 

57.19-91 Mandatory. Hoist operators 
shall accept hoisting instructions only by 
the regular signaling system unless it is out 
of order. In such an event, and during other 
emergencies, the hoist operator shall accept 
instructions to direct movement of the con- 
veyances only from authorized persons. 

57.19-92 Mandatory. A method shall be 
provided to signal the hoist operator from 
cages or other conveyances at any point in 
the shaft. 

57.19-93 Mandatory. A standard code of 
hoisting signals shall be adopted and used 
at each mine. The movement of a shaft con- 
veyance on a "one bell" signal is prohibited. 

57.19-94 Mandatory. A legible signal code 
shall be posted prominently In the hoist 
house within easy view of the hoistmen, and 
at each place where signals are given or re- 
ceived. 

57.19-95 Mandatory. Hoisting signal de- 
vices shall be positioned within easy reach 
of persons on the shaft bottom or constant- 
ly attended by a person stationed on the 
lower deck of the siniing platform. 

57.19-96 Mandatory. Any person respon- 
sible for receiving or giving signals for cages, 
skips, and mantrips when men or materials 
are being transported shall be familiar with 
the posted signaling code. 



Mine Classification 

57.21-1 Mandatory. A mine shall be 
deemed gassy, and thereafter operated as a 
gassy mine, if: 

(a) The State in which the mine is located 
classifies the mine as gassy; or 

(b) Flammable gas emanating from the 
orebody or the strata surrounding the ore- 
body has been ignited in the mine; or 

(c) A concentration of 0.25 percent or 
more, by air analysis, of flammable gas ema- 
nating only from the orebody or the strata 
surrounding the orebody has been detected 
not less than 12 inches from the back, face, 
or ribs in any open workings; or 

(d) The mine is connected to a gassy mine. 

57.21-2 Mandatory. Flammable gases de- 
tected only while unwatering mines or 
flooded sections of mines or during other 
mine reclamation operations shall not be 
used to permanently classify a mine gassy. 
During such periods that any flammable gas 
is present in the mine, the affected areas of 
the mine shall be operated in accordance 
with appropriate standards in this Section 
57.21. 



Ventilation 
57.21-20 Mandatory. Main fans shall be: 



§ 57.20 Miscellaneous. 

General— ScitPACE and Underground 



(f) Provided with an automatic signal 
device to give warning or alarm should the 
fan system malfunction. The signal device 
shall be so located that it can be seen or 
heard by a responsible person at all times 
when persons are underground. 



57.20-32 Mandatory. Telephones or other 
two-way communication equipment with 
instructions for their use shall be provided 
for communication from underground oper- 
ations to the surface. 

[34 FR 12517, July 31, 1969, as amended at 
35 FR 3677, Feb. 25, 1970; 42 FR 29424, June 
8, 1977; 42 FR 57044, Oct. 31, 1977; 44 FR 
31919, June 1, 1979; 44 FR 48535, Aug. 17, 
1979] 

§ 57.21 Gassy mines. 

Gassy mines shall be operated in accord- 
ance with all mandatory standards in this 
part. Such mines shall also be operated in 
accordance with the mandatory standards 
in this section. The standards in this section 
apply only to underground operations. 



57.21-29 Mandatory. Booster fans shall 
be: 

(a) Provided with an automatic signal 
device to give warning or alarm should the 
fan system malfunction. The signal device 
shall be so located that it can be seen or 
heard by a responsible person at all times 
when persons are underground. 

(b) Equipped with a device that automati- 
cally deenergizes the power in affected 
active workings should the fan system mal- 
function. 

(c) Provided with air locks, the doors of 
which open automatically should the fan 
stop. 

(d) Equipped with two sets of controls ca- 
pable of starting, stopping, and reversing, 
the fans. One set of controls shall be located 
at the fans. A second set of controls shall be 
at another location remote from the fans. 



199 



PART 75— MANDATORY SAFETY 
STANDARDS— UNDERGROUND 
COAL MrNES 



§ 75.321 Stoppage of fans, plans. 

[Statutory Provisions] 

Each operator shall adopt a plan on 
or before May 29, 1970, which shall 
provide that when any mine fan stops, 
immediate action shall be taken by the 
operator or his agent (a) to withdraw 
all persons from the working sections, 
(b) to cut off the power in the mine in 
a timely manner, (c) to provide for res- 
toration of power and resumption of 
work if ventilation is restored within a 
reasonable period as set forth in the 
plan after the working places and 
other active workings where methane 
is likely to accumulate are reexamined 
by a certified person to determine if 
methane in amounts of 1.0 volume per 
centum or more exists therein, and (d) 
to provide for withdrawal of all per- 
sons from the mine if ventilation 
cannot be restored within such reason- 
able time. The plan and revisions 
thereof approved by the Secretary 
shall be set out in printed form and a 
copy shall be furnished to the Secre- 
tary or his authorized representative. 



§75.516-2 Communication wires and 
cables; installation; insulation; support. 

(a) All communication wires shall be 
supported on insulated hangers or in- 
sulated J-hooks. 

(b) All communication cables shall 
be insulated as required by § 75.517-1, 
and shall either be supported on insu- 
lated or uninsulated hangers or J- 
hooks, or securely attached to messen- 
ger wires, or buried, or otherwise pro- 
tected against mechanical damage in a 
manner approved by the Secretary or 
his authorized representative. 

(c) All communication wires and 
cables installed in track entries shall, 
except when a communication cable is 
buried in accordance with paragraph 
(b) of this section, be installed on the 
side of the entry opposite to trolley 
wires and trolley feeder wires. Addi- 
tional insulation shall be provided for 
communication circuits at points 
where they pass over or under any 
power conductor. 

(d) For purposes of this section, com- 
munication cable means two or more 
insulated conductors covered by an ad- 
ditional abrasion-resistant covering. 

t38 FR 4975, Feb. 2.3, 1973] 

§ 75.517 Power wires and cables; insula- 
tion and protection. 

[Statutory Provisions] 

Power wires and cables, except trol- 
ley wires, trolley feeder wires, and 
bare signal wires, shall be insulated 
adequately and fully protected. 

§ 75.517-1 Power wires and cables; insula- 
tion and protection. 

Power wires and cables installed on 
or after March 30, 1970, shall have in- 
sulation with a dielectric strength at 
least equal to the voltage of the cir- 
cuit. 



§ 75.508-1 Mine tracks. 

When mine track is used as a con- 
ductor of a trolley system, the location 
of such track shall be shown on the 
map required by § 75.508, with a nota- 
tion of the number of rails and the 
size of such track expressed in pounds 
per yard. 



§ 75.521 Lightning arresters; ungrounded 
and exposed power conductors and 
telephone wires. 

Each ungrounded, exposed power 
conductor and each ungrounded, ex- 
posed telephone wire that leads under- 
ground shall be equipped with suitable 
lightning arresters of approved type 
within 100 feet of the point where the 
circuit enters the mine. Lightning ar- 
resters shall be connected to a low 
resistance grounding medium on the 
surface which shall be separated from 
neutral grounds by a distance of not 
less than 25 feet. 



[38 FR 4975, Feb. 23, 1973] 



200 



§75.701-4 Grounding wires; capacity of 
wires. 
Where grounding wires are used to 
ground metallic sheaths, armors, con- 
duits, frames, casings, and other me- 
tallic enclosures, such grounding wires 
will be approved if: 

(a) The cross-sectional area (size) of 
the grounding wire is at least one-half 
the cross-sectional area (size) of the 
power conductor where the power con- 
ductor used is No. 6 A.W.G., or larger. 

(b) Where the power conductor used 
is less than No. 6 A.W.G., the cross- 
sectional area (size) of the grounding 
wire is equal to the cross-sectional 
area (size) of the power conductor. 



§ 75.1003-1 Other requirements for guard- 
ing of trolley wires and trolley feeder 
wires. 

Adequate precaution shall be taken 
to insure that equipment being moved 
along haulageways will not come in 
contact with trolley wires or trolley 
feeder wires. 

§ 75.1003-2 Requirements for movement 
of off-track mining equipment in areas 
of active workings where energized 
trolley wires or trolley feeder wires are 
present; pre-movement requirements; 
certified and qualified persons. 



and such electric power can be sup- 
plied only from inby the equipment 
being moved or transported, power 
may be supplied from inby such equip- 
ment provided a miner with the means 
to cut off the power, and in direct 
communication with persons actually 
engaged in the moving or transporting 
operation, is stationed outby the 
equipment being moved. 

(2) The settings of automatic circuit 
interrupting devices used to provide 
short circuit protection for the trolley 
circuit shall be reduced to not more 
than one-half of the maximum cur- 
rent that could flow if the equipment 
being moved or transported were to 
come into contact with the trolley 
wire or trolley feeder wire; 

(3) At all times the unit of equip- 
ment is being moved or transported, a 
miner shall be stationed at the first 
automatic circuit breaker outby the 
equipment being moved and such 
miner shall be: (i) In direct commimi- 
cation with persons actually engaged 
in the moving or transporting oper- 
ation, and (ii) capable of communicat- 
ing with the responsible person on the 
surface required to be on duty in ac- 
cordance with § 75.1600-1 of this part; 

(4) Where trolley phones are utilized 
to satisfy the requirements of para- 
graph (f)(3) of this section, telephones 
or other equivalent two-way communi- 
cation devices that can readily be con- 
nected with the mine communication 
system shall be carried by the miner 
stationed at the first automatic circuit 
breaker outby the equipment being 
moved and by a miner actually en- 
gaged in the moving or transporting 
operation; and. 



(f) A minimum vertical clearance of 
12 inches shall be maintained between 
the farthest projection of the unit of 
equipment which is being moved and 
the energized trolley wires or trolley 
feeder wires at all times during the 
movement or transportation of such 
equipment; provided, however, that if 
the height of the coal seam does not 
permit 12 inches of vertical clearance 
to be so maintained, the following ad- 
ditional precautions shall be taken: 

(l)(i) Except as provided in para- 
graph (f)(l)(ii) of this section electric 
power shall be supplied to the trolley 
wires or trolley feeder wires only from 
outby the unit of equipment being 
moved or transported, (ii) Where 
direct current electric power is used 



§ 75.1402 Communication between shaft 
stations and hoist room. 



[Statutory Provisions] 

There shall be at least two effective 
methods approved by the Secretary of 
signaling between each of the shaft 
stations and the hoist room, one of 
which shall be a telephone or speaking 
tube. 



§ 75.1402-1 Communication between shaft 
stations and hoist room. 

One of the methods used to commu- 
nicate between shaft stations and the 
hoist room shall give signals which can 
be heard by the hoisting engineer at 
all times while men are underground. 



201 



§ 75.1402-2 Tests of signaling systems. 

Signaling systems used for conununi- 
cation between shaft stations and the 
hoist room shall be tested daily. 



Subpart Q — Communications 

§ 75.1600 Communications. 

[Statutory Provisions] 

Telephone service or equivalent two- 
way communication facilities, ap- 
proved by the Secretary or his author- 
ized representative, shall be provided 
between the surface and each landing 
of main shafts and slopes and between 
the surface and each working section 
of any coal mine that is more than 100 
feet from a portal. 

§ 75.1600-1 Communication facilities; 

main portals; installation require- 
ments. 
A telephone or equivalent two-way 
communication facility shall be locat- 
ed on the surface within 500 feet of all 
main portals, and shall be installed 
either in a building or in a box-like 
structure designed to protect the facil- 
ities from damage by inclement weath- 
er. At least one of these commimica- 
tion facilities shall be at a location 
where a responsible person who is 
always on duty when men are under- 
ground can hear the facility and re- 
spond immediately in the event of an 
emergency. 

[38 FR 29999, Oct. 31, 1973] 

§ 75.1600-2 Communication facilities; 

working sections; installation and 
maintenance requirements; audible or 
visual alarms. 

(a) Telephones or equivalent two- 
way communication facilities provided 
at each working section shall be locat- 
ed not more than 500 feet outby the 
last open crosscut and not more than 
800 feet from the farthest point of 
penetration of the working places on 
such section. 



(b) The incoming communication 
signal shall activate an audible alarm, 
distinguishable from the surrounding 
noise level, or a visual alarm that can 
be seen by a miner regularly employed 
on the working section. 

(c) If a communication system other 
than telephones is used and its oper- 
ation depends entirely upon power 
from the mine electric system, means 
shall be provided to permit continued 
communication in the event the mine 
electric power fails or Is cut off; pro- 
vided, however, that where trolley 
phones and telephones are both used, 
an alternate source of power for the 
trolley phone system is not required. 

(d) Trolley phones connected to the 
trolley wire shall be grounded in ac- 
cordance with Subpart H of this part. 

(e) Telephones or equivalent two- 
way communication facilities shall be 
maintained in good operating condi- 
tion at all times. In the event of any 
failure in the system that results in 
loss of communication, repairs shall be 
started immediately, and the system 
restored to operating condition as soon 
as possible. 

[38 PR 29999, Oct. 31, 1973] 



§ 75.1713-2 Emergency communications; 
requirements. 

(a) Each operator of an underground 
coal mine shall establish and maintain 
a communication system from the 
mine to the nearest point of medical 
assistance for use in an emergency. 

(b) The emergency communication 
system required to be maintained 
under paragraph (a) of this § 75.1713-2 
may be established by telephone or 
radio transmission or by any other 
means of prompt commxinication to 
any facility (for example, the local 
sheriff, the State highway patrol, or 
local hospital) which has available the 
means of communication with the 
person or persons providing emergen- 
cy medical assistance or transporta- 
tion in accordance with the provisions 
of § 75.1713-1. 



202 



APPENDIX C. —EQUIPMENT SUPPLIERS 



Pager Phones (And Associated Equipment) 

Appalachian Electronics 
801 West Monroe Ave. 
Ronceverte, WV 24970 

ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

CSE Mine Service Co. 
600 Seco Rd. 
Monroevllle, PA 15146 

Fairmont Supply Co. 

Box 501 

Washington, PA 15301 

FEMCO (See National Mine Service Co.) 

Gal-Tronlcs Corp. 
P.O. Box 31-T 
Reading, PA 19603 

Harrison R. Cooper Systems, Inc. 

AME Box 22014 

Salt Lake City, UT 84122 

JABCO (See Schroeder Brothers Corp.) 

Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 

National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 

Prelser/Mlneco 
Jones & Oliver Sts. 
St. Albans, WV 25177 

Pyott-Bonne, Inc. 
P.O. Box 809 
Tazewell, VA 24651 

Schroeder Brothers Corp. 

Nlchol Ave. 

Box 72 

McKees Rocks, PA 15136 



Wlnster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Carrier Phones 

American Mine Research, Inc. 
P.O. Box 1628 
Bluefleld, WV 24701 

ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

CSE Mine Service Co. 
600 Seco Rd. 
Monroevllle, PA 15146 

Fairmont Supply Co. 

Box 501 

Washington, PA 15301 

FEMCO (See National Mine Service Co.) 

Harrison R. Cooper Systems, Inc. 

AMF Box 22014 

Salt Lake, UT 84122 

Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 

National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 

Hoist Communications 

ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

Fairmont Supply Co. 

Box 501 

Washington, PA 15301 



203 



FEMCO (See National Mine Service Co.) 

Harrison R. Cooper Systems, Inc. 

AMF Box 22014 

Salt Lake City, UT 84122 

Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 

National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 

Republic Wire and Cable 
P.O. Box 352 
Flushing, NY 11352 

Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

PABX and Multiplex Equipment 



Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Intercoms 

ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

Executone, Inc. 

Dept. TR-77 

Long Island City, NY 11101 

FEMCO (See National Mine Service Co.). 

Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 

National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 



Anaconda Telecommunications 
305 North Muller 
Anaheim, CA 92801 

Essex Group 

800 East Garfield Ave. 

Decatur, IL 62525 

Executone, Inc. 

Dept. TR-77 

Long Island City, NY 11101 

Phelps Dodge Communication Co. 
5 Corporate Park Dr. 
White Plains, NY 10604 

Pulsecom Div. 
Harvey Hubbell, Inc. 
5714 Columbia Pike 
Falls Church, VA 22041 

Reliable Electric Co. 
11333 West Addison 
Franklin Park, IL 60131 

Til Industries, Inc. 
100 North Strong Ave. 
Lindenhurst, NY 11757 



Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Radio Pocket Pagers 

Executone, Inc. 

Dept. TR-77 

Long Island City, NY 11101 

FEMCO (See National Mine 
Service Co.) 

General Electric Co. , Mobile Radio Dept. 
P.O. Box 4197 
Lynchburg, VA 24502 

National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 

Leaky Feeder Equipment 

Andrew Corp. 

10500 West 153d St. 

Orland Park, IL 60462 



204 



Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Mobile Radio Equipment 

Fairmont Supply Co . 

Box 501 

Washington, PA 15301 

General Electric Co., Mobile 

Radio Dept. 
P.O. Box 4197 
Lynchburg, VA 24502 



ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

FEMCO (See National Mine Service Co.) 

General Electric Co., Mobile Radio Dept, 
P.O. Box 4197 
Lynchburg, VA 24502 

General Equipment & Manufacturing 

Co. , Inc. 
3300 Fern Valley Rd. 
P.O. Box 13226 
Louisville, KY 40213 



Motorola Communications & Electronics 
1301 East Algonquin Rd. 
Schaumburg, IL 60196 



Mag-Con, Inc. 
1626 Terrace Dr. 
St. Paul, MN 55113 



Lee Engineering 

2025 West Wisconsin Ave. 

Milwaukee, WI 53201 



Mine Safety Appliances Co. 
600 Penn Center Blvd. 
Pittsburgh, PA 15235 



Phelps Dodge Communication Co. 
5 Corporate Park Dr. 
White Plains, NY 10604 



National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 



Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Closed Circuit Television 

Midwest Telecommunications Div. , 
Midwest Corp. 
300 T First Ave. 
Nitro, WV 25143 

Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 

Remote Control and Monitor Equipment 

American Mine Research, Inc. 
P.O. Box 1628 
Bluefield, WV 24701 

BIF Accutel Inc. 
1339 Lawrence Dr. 
Newbury Park, CA 91320 



Notifier of Western Penn. Inc. 
3460 Babcock Blvd. 
Pittsburgh, PA 15237 

Pace Transducer Co., Div. of 

C. J. Enterprises 
P.O. Box 834 
Tarzana, CA 91356 

Pulsecom Div. 
Harvey Hubbell, Inc. 
5714 Columbia Pike 
Falls Church, VA 22041 

Py o 1 1 -Bonne , Inc . 
P.O. Box 809 
Taxewell, VA 24651 

RFL Industries, Inc. 
Boonton, NJ 07005 

Stevens International Inc. 

P.O. Box 619 

Kennett Square, PA 19348 



205 



Winster Engineering Ltd. 
Manners Ave. 
Ilkeston, Derbyshire 
United Kingdom 



Environmental Sensors 



Methane: 



Mine Safety Appliances Co. 
201 North Braddock. Ave. 
Pittsburgh, PA 15208 

National Mine Service Co. 
300 Koppers Bldg. 
Pittsburgh, PA 15216 

Bacharach Instrument Co. 
625 Alpha Dr. 
Pittsburgh, PA 15238 

CSE Mine Service Co. 
2000 Eldo Rd. 
Monroeville, PA 15146 

Preiser /Mine CO 

Jones and Oliver Sts. 

St. Albans, WV 25177 

Appalachian Electronics Instruments 
810 West Monroe Ave. 
Ronceverte, VA 24970 

American Mine Research, Inc. 
P.O. Box 1628 
Bluefield, WV 24701 

Carbon monoxide : 

Mine Safety Appliances 
201 North Braddock Ave. 
Pittsburgh, PA 15208 



Edmont -Wilson 
1300 Walnut St. 
Coshocton, OH 43812 

Mine Safety Appliances Co. 
201 North Braddock Ave. 
Pittsburgh, PA 15208 

Survivair Div. of U.S. Divers 
3323 West Warner Ave. 
Santa Ana, CA 91776 

Teledyne Analytical Instruments 

333 West Mission 

San Gabriel, CA 91776 

Oxides of nitrogen: 

Energetics Sciences 
85 Executive Blvd. 
Elmsford, NY 10523 

Air flow sensors: 

Alnor Instrument Co. 
7301 North Caldwell Ave. 
Niles, IL 60648 

J-Tec Associates, Inc. 
317 Seventh Ave. SE 
Cedar Rapids, lA 52401 

Taylor Instrument 
Consumer Products Div. 
Arden, NC 28704 

Atmospheric pressure: 

Pace Transducer Co., Div. of 

C. J. Enterprises 
P.O. Box 834 
Tarzana, CA 91356 



Energetics Sciences 
85 Executive Blvd. 
Elmsford, NY 10523 



Leeds and Northrup Co. 

Dept. MD337 

North Wales, PA 19454 



Oxygen : 



Beckman Instruments Inc. 

3900 River Rd. 

Schiller Park, IL 60176 



206 



Seismic Equipment 

Pace Transducer Co., Dlv. of 

C. J. Enterprises 
P. 0. Box 834 
Tarzana, CA 91356 

Consultants 

Arthur D. Little, Inc. 
25 Acorn Park 
Cambridge, MA 02140 

Advance Mining Services 

616 Beatty RD. Industrial Court 

Monroeville, PA 15146 



Pyott -Bonne, Inc. 
P.O. Box 809 
Tazewell, VA 24651 

Corma Resources 
2857 Mount Vernon SE 
Cedar Rapids, lA 52403 

U.S. Bureau of Mines 
4800 Forbes Avenue 
Pittsburgh, PA 15213 

Winster Engineering Ltd. 
Manners Avenue 
Ilkeston, Derbyshire 
United Kingdom 



ComTrol Corp. 
500 Penna. Ave. 
Irwin, PA 15642 

CSE Mine Service Co. 
600 Seco Rd. 
Monroeville, PA 15146 

Fairmont Supply Co. 

Box 501 

Washington, PA 15301 

General Electric Co., Mobile Radio Dept. 
P.O. Box 4197 
Lynchburg, VA 24502 

Midwest Telecommunications Dlv. , 

Midwest Corp. 
300 T First Ave. 
Nitro, WV 25143 

Mineral Services Inc. 
1276 West Third St. 
Cleveland, OH 44113 

National Coal Board 

Mining Research and Development 

Establishment 
Stanhope Bretby 
Burton Upon Trent DEISOQD 
United Kingdom 



Fire Detection Devices 

ADT Co. , Inc. 

155 Sixth Ave. 

New York, NY 10013 

The Ansul Co. 
One Stanton St. 
Marinette, WI 54143 

B. & B. Electric Manufacturing Co. 
Seward, PA 15954 

Gammaflex Corp. 

821 Michael Faraday Dr. 

Res ton, VA 22070 

JABCO 

Schroeder Brothers Corp. 

P.O. Box 72 

Nlchol Ave. 

McKees Rocks, PA 15136 

McJunkin Corp. 
P.O. Boc 2473 
1400 Hansford St. 
Charleston, WV 25311 

Mine Safety Appliances Co. 
201 North Braddock Ave. 
Pittsburgh, PA 15203 



National Mine Service Co. 
4900/600 Grant St. 
Pittsburgh, PA 15219 



National Mine Service Co. 
3000 Koppers Bldg. 
436 Seventh Ave. 
Pittsburgh, PA 15219 



207 



Notifier of Western Pennsylvania 
3283 Babcock Blvd. 
Pittsburgh, PA 15237 

Prieser 

Jones and Oliver Sts. 

St. Albans, WV 25177 

Pyott-Boone, Inc. 
P.O. Box 809 
Tazewell, VA 24651 

Southern Engineering and Equipment Co. 
P.O. Drawer 329 
95 Third St. , NE 
Graysville, AL 35073 



General Cable Corp. 
600 Reed Rd. 
Broomall, PA 19008 

Industrial Component Inc. 

342 Madison Ave. 

Suite 702 

New York, NY 10017 

Okonite Co. 
100 Hilltop Rd. 
Ramsey, NJ 07446 



Figure-8 Communication Cable 

Delphi Wire & Cable 
700 Carpenters Crossing 
Folcroft, PA 19032 



208 



APPENDIX D.— GLOSSARY OF TERMS 



Analog 



A method of generating or transmitting information that is repre- 
sented by a continuous (as opposed to digital) voltage or current 
that is proportional to the information. 



Angstrom 



A unit of length. Usually used to measure the wavelength of light 
or other radiation. One angstrom is equal to one hundred-millionth 
of a centimeter. 



AM 



Abbreviation for "amplitude modulation. " Modulation in which the 
amplitude of the information waveform modulates the amplitude of a 
carrier wave. 



Attenuation 



Balance point 



Bandwidth 



The decrease in signal strength during its transmission from one 
point to another. Attenuation is usually expressed in decibels. 

In an electronic bridge circuit, the point at which the electrical 
resistances in both branches of the network are the same. 

The difference (in cycles per second) between the highest and lowest 
frequency components required for the adequate transmission of 
information. 



Baseband 



The original frequency band (before modulation) of a signal. Usu- 
ally refers to the baseband of an audio or voice signal, which is 
approximately 300 to 5,000 Hz. 



Binary 



A digital numbering system with the base 2. In a binary system 
there are only two possibilities for each digit, selection, choice, 
or condition. For example, a simple switch is a binary device since 
it is either open or closed. 



Bridge 



An electrical bridge circuit is a network arranged so that voltage 
or current in one branch of the circuit may be measured by adjusting 
components in another branch of the circuit. 



CATV 



Abbreviation for "community antenna television,' 
cable television. 



commonly known as 



Characteristic 
impedance 



Pertaining to transmission lines. For a uniform and infinitely long 
line, it is the ratio of applied voltage to current induced at a 
given frequency. It is measured in ohms and usually designated as 
Zo. For maximum signal transfer, the Zo of a line should equal the 
Zo of a source and load. 



CO Abbreviation for "central office." Refers to the telephone com- 

pany's central office. 

Cross talk Cross-coupling or interference between speech channels or wire 
pairs. 



dB 



Abbreviation of "decibel," a unit that represents the ratio between 
two amounts of power on a logarithmic scale. A value of +3 dB in- 
dicates a doubling of power, while -3 dB is a halving of power. 



209 



dBm 



The normal signal level in a pager phone is about 1 milliwatt 
(1 mW). The designation dBm is used to indicate this 1-mW refer- 
ence level. Thus, +3 dBm is 3 dB above the reference (2 mW) and 
-3 dBm is 3 dB below the reference (0.5 mW). 



Demodulation 



DTMF 



A device that receives a carrier wave and recovers or "reconstructs" 
the original voice or information signal from the carrier wave. 

Abbreviation for "dual-tone multif requency ." A phone signaling 
method in which each digit dialed is converted to a dual-tone signal 
that will be recognized by the telephone office or PABX switching 
equipment. These control tones can be heard in the earpiece when 
dialing on many pushbutton phones. 



Electromagnetic 
Encoder 



Having both electric and magnetic properties. 

A unit that produces coded output combinations depending upon the 
specific input selected. 



FDM 



Abbreviation for "frequency-division multiplexing." A process in 
which two or more signals are sent over a common path by sending 
each one in a different frequency band. 



Feedback 



In a transmission system, or electrical device, the returning of a 
fraction of the output signal to the input. 



FM 



Abbreviation for "frequency modulation." Modulation in which the 
amplitude of the information waveform modulates the frequency of a 
carrier signal. 



FSK 



Abbreviation for "frequency-shift keying." A form of FM in which a 
binary code is transmitted by switching a carrier signal between two 
different frequencies. 



Ge op hone 



A device used to detect seismic vibrations or Shockwaves in the 
earth. 



Hall effect In a conductor located in a magnetic field that is perpendicular to 
the direction of current, the production of a voltage perpendicular 
to both the current and the magnetic field. 



Handset 
Headset 



A receiver-transmitter held by hand. 

A receiver-transmitter that can be attached to the person to allow 
"hands-free" operation. 



Hybrid 



A circuit or communications system that is made up of two or more 
dissimilar systems. 



Hz 



Abbreviation for Hertz . A unit of frequency equal to 1 cycle per 
second. 



Impedance The total opposition (reactance plus resistance) that a circuit or 
transmission line offers to the flow of electrical current. 



210 



Inductively 
coupled 



Joule heating 
Leaky feeder 



Method of inducing a signal into one conductor or wire from another 
conductor even though there may be no mechanical connection between 
the two conductors. (The magnetic field set up in the space around 
a conductor carrying alternating current will induce a signal in 
other nearby conductors.) 

In an electrical circuit, the heat produced by the flow of current 
in the circuit. 

A specially designed coaxial cable that allows radio signals to leak 
into or out of the cable so that they may be picked up by radio 
transceivers. 



LED 

Magnetic field 

Magneto 

Milliammeter 

Modem 

Modulator 
Monochromatic 
Multiplexed 
PABX 

PAM 



Parasitic 
coupling 



PBX 



PCM 



Abbreviation for "light-emitting diode." A solid state electronic 
device that emits light when a current flows through it. 

The region surrounding a magnet or a conductor through which current 
is flowing. 

An ac generator for producing ringing signals. 

An electric current meter calibrated in milliamperes. 

A device that is both a modulator and a demodulator. A modem is a 
two-way device that both modulates (transmits) and receives (demodu- 
lates) a signal. 

A device that modulates a voice or information signal and transmits 
the resulting carrier wave. 

A signal or beam of light consisting of a single wavelength or of a 
very small range of wavelengths. 

The simultaneous transmission of two or more signals using a single 
transmission path or wire. 

Abbreviation for "private automatic branch exchange." A private 
branch exchange in which automatically controlled switches make con- 
nections between the phones in the system. 

Abbreviation for "pulse amplitude modulation." Modulation in which 
the value or amplitude of each sample of the information waveform 
modulates the amplitude of a pulse carrier. 

The coupling of radio waves or electrical signals from one wire or 
medium to another with the result that the signal strength in the 
first conductor is decreased. 

Abbreviation for "private branch exchange. " A private manual tele- 
phone exchange requiring an operator at a switchboard to make con- 
nections between the phones. 

Abbreviation for "pulse coded modulation." Modulation in which the 
value or amplitude of each sample of the information waveform is 
quantitized and transmitted as a digital binary code. 



211 



PDM 



Abbreviation for "pulse duration modulation." Modulation in which 
the value or amplitude of each sample of the information waveform 
modulates the duration, or "width," of a pulse. 



Piezoelectric 



The property of certain crystals or materials that produce a voltage 
when subjected to mechanical stress. 



Potentiometer 



An electromechanical device with a sliding contact on a resistor. 
Movement of the sliding contact changes the electrical resistance of 
the circuit and allows the electronics to sense the position of the 
sliding contact. 



PPM 



Abbreviation for "pulse position modulation." Modulation in which 
the value or amplitude of each sample of the information waveform 
modulates the position in time of a pulse. 



Propagation The travel of electromagnetic (radio) or sound waves through a 
medium. 



PSK 



Abbreviation for "phase shift keying." A form of FM in which a 
binary code is transmitted by shifting the phase of a carrier 
signal. 



Q The "Q" of an ac circuit is the ratio of its reactance to its re- 
sistance. The voltage developed across the reactance is usable sig- 
nal, but the voltage developed across the resistance subtracts 
from the signal. Thus, a high Q indicates an efficient, low-loss 
ac circuit. 

Reactance The opposition to the flow of alternating current (ac) . Capacitive 
reactance (Xq) is the opposition offered by capacitors, and induc- 
tive reactance (Xl) is the opposition offered by a coil or other 
inductance. 



rf 



Abbreviation for radiof requency. Any frequency at which electromag- 
netic radiation of energy (radio waves) is possible. 



RFI Radio frequency interference. 

Reluctance The resistance of a magnetic path to the flow of magnetic line of 
force. Aluminum has a high reluctance; iron has a low reluctance. 



Repeater 



A device that detects or receives a signal and rebroadcasts the same 
signal. 



Resistance The opposition to the flow of direct current (dc) . The unit of re- 
sistance is the ohm. 



Resonate 
Simplex 



To bring to resonance; to tone. 

A communication system, or other device, that operates in only one 
direction (either transmit or receive) at a time. 



212 



Sine wave 



SWR 



Synchronize 
TDM 



Transducer 

Transceiver 

Tuned voltmeter 

UHF 

Ultrasonic 

vf 

VHF 
Vortex 

Waveguide 



The wave form corresponding to a pure, single-frequency 
oscillation. 

Abbreviation for "standing wave ratio." On a transmission line or 
antenna element the current and voltage set up by waves traveling in 
the opposite direction are characterized by the presence of a number 
of stationary maximum and minimum points in the distribution curve. 
SWR is the ratio of the maximum to minimum current or voltage of 
these stationary waves. 

To maintain one operation (or signal) in step with another. 

Abbreviation for "time-division multiplexing." A process by which 
two or more channels of information are transmitted over the same 
link by allocating a different time interval for the transmission of 
each channel. 

A device that converts energy from one form to another. A seismic 
transducer, for instance, converts seismic shock waves into elec- 
trical signals. 

A device that is both a transmitter and a receiver. A two-way CB 
radio is a transceiver. 

A voltmeter that has been tuned to detect voltage levels or signal 
strengths at specific frequencies. 

Ultra high frequency, 300 to 3,000 MHz 

Having a frequency above that of audible sound. 

Abbreviation for voice frequency (same as audio frequency). The 
frequencies corresponding to speech or other audible sound wave. 

Very high frequency, 30 to 300 MHz. 

A whirlpool or eddy caused by a fluid or gas moving past an 
obstruction. 

A hollow, round or rectangular pipe (or tunnel), used as a trans- 
mission line for signaling. 



INT.-BU.OF MINES,PGH.,P A. 26E07 




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